Explaining Time to Kids

Don’t forget that the deadline for this year’s Flame Challenge is coming up. Your mission: to explain “Time” to a group of 11-year-olds, who will be sternly judging your work. Get your submissions in by March 1, either video or written (less than 300 words).

Here’s my attempt. (Just given the likely number of entries, winning seems like a long shot, so I don’t mind encouraging other submissions or giving away all my best lines.) 300 words is hard, and aiming squarely at 11-year-olds who are judging a bunch of submissions is also no easy feat. But it’s good practice. I personally first fell in love with science when I was 10, so 11-year-olds are a great audience to aim at.

Admittedly the definitions I propose below could be accused of being circular, but without using technical jargon I think it’s appropriate to aim for intuitive understanding rather than perfect rigor.


Time is not hard to understand! How time works can be tricky, but time itself isn’t that mysterious.

We live in a world full of stuff. Chairs, trees, planets, stars, all kinds of things. This stuff is spread throughout space–everything has a location somewhere or another. And all this stuff, at various positions in space, happens over and over again, slightly differently each time. Things move, age, transform. Planets orbit stars, animals eat and sleep, people play and fight and think and learn. The universe doesn’t sit still.

Time is the label we stick on different moments in the life of the world. There is the universe at 2 p.m. July 1st 2013, the universe at 2:01 p.m., and so on. Just like a page number tells you where you are in a book, time tells you when you are in the universe. Moments of time are pages in the book of the universe.

We can measure time using clocks and calendars—things that repeat themselves in a predictable way. Every time the Earth revolves around the Sun, it rotates around its axis about 365 times. Every time the little hand goes around a clock dial, we can be sure the big hand goes around twelve times.

Time gets mysterious when we think about past, present, and future. We can remember what happened yesterday, but we can’t remember tomorrow. It seems obvious, but why is it true? Why does everyone – everyone! – get born young, and then grow old? We can choose what to do next in our lives, but we can’t un-choose events in the past, things that have already happened. The past is in the books, but the future remains to be shaped. Let’s hope we choose wisely!

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23 Responses to Explaining Time to Kids

  1. Pulsar says:

    “Every time the Earth revolves around the Sun, it rotates around its axis about 365 times.”

    Actually, it rotates about 366 times, due to the difference between a solar day and a siderial (or more correctly, a stellar) day.

  2. Sean Carroll says:

    366 is, in fact, “about 365.” But in 300 words I can’t explain the solar/siderial difference, nor do I want to baffle the 11-year-olds.

  3. Personally, I thought it was great. Really nice, really clear description.

    If, when I was I young, someone had explained time to me that clearly I never would have been so late too school.

    Bravo. And good luck 🙂

  4. Crapbag says:

    It’s good, but it seems more like proto-philosophy than proto-physics. Perhaps you could draw attention to the amazing fact that questions about time are indeed questions for *scientists*. In other words, if you really want to learn about time, you had better plan on becoming a physicist.

  5. Navneeth says:

    The problem I feel with this particular challenge, as opposed to the previous one, is that even adults don’t have a good or complete understanding of what time is, unlike the atomic mechanics and chemistry of a flame (obligatory link).

  6. Durgadas Datta says:

    Time can not be understood without a discussion of entropy in the context of second law.Einstein wrongly developed ideas of time-space convolution and declared time a dimension but time is entropical reflection of space. How it happens in the energy gradient of space under an order of gravitoetherton soup is being studied in CERN.

  7. Tom Clark says:

    “The past is in the books, but the future remains to be shaped.”

    Actually, the future, like the present and past, is fixed as part of the 4-D spacetime block universe, no? The future is *epistemically* open since we don’t know what it is, but not metaphysically.

    Of course we can’t tell kids that, but I think we can tell adults. It’s the same message as not having contra-causal free will.

  8. Do these modern kids actually know how an analog clock works?

  9. bob says:

    Your second paragraph sounds a lot like “time is what keeps everything from happening at once” (Ray Cummings, in various s.f. novellas in the 1920s – and repeated by John Wheeler!).

  10. Neil says:

    Bob.

    That is okay. Frankly, I’ve never heard a better description.

  11. Tony Rz says:

    I have read where, of course for those who believe in God, that God sees all time in a single instant. Very interesting.

  12. Antonio Sanchez says:

    If we try explain time to kids using the famous arrow of time originated at the Big Bang, we would be disqualified.
    Not to mention describing us as biological machines in continuous slow combustion to explain why we can´t reverse time.
    Entropy, thermodynamics are also forbidden words…Not many options left to be original.
    I could use a metaphor of time as a wave that arises suddenly in a silent (almost) motionless cosmic ocean. We humans are just surfers. When the time-wave finally disappears, the infinite cosmos will engulf us like a tear in the ocean.

  13. Ed Shafer says:

    I am not a scientist, therefore I could not submit this explanation: Imagine tonight at midnight every motion of everything in the universe stopped. Every object, every person (including you), every molecule, every atom, every particle within every atom, every force between every object and every particle simply stopped moving. Every child has imagined this. Now imagine that everything started up again. Then ask the question: How long had everything been stopped? There is no answer because there had been nothing moving to “keep time”. Therefore, time is how we keep track of motion or change. Things move, and humans use some of that movement to keep time.

  14. Ramon Negron says:

    I was trained as a physicist and taught college physics for a number of years a while back. I used to explain time as a fourth dimention of space, something my students would have at least heard about.

    I would start with the notion that every physical dimention is ‘made’ from lower level dimentions placed next to each other. You start with a point, zero dimentions. A collection of points next to each other forms a line, one dimention. A collection of lines then forms a plane. A collection of planes forms a volume, ie our usual three dimentional space. So what is time?

    Time is a collection of volumes, or spaces ‘next to’ each other.

    The space now, ‘next to’ the space now, ‘next to’ the space a minute from now, and so on.

    It worked well I think, and still does not seem out of place.

    To Ed Shafer: I liked your take. There is a very nice illustration of that explanation in a short story by Jorge Luis Borges titled “El milagro secreto” (http://www.ciudadseva.com/textos/cuentos/esp/borges/milagro.htm) in english “The secret miracle” (http://hadideeb.com/journal/2008/3/4/el-milagro-secreto.html).

    Sometimes I had my students read that short story and try to answer some questions about the flow of time on it prior to my lecture on time. That worked well with really bright kids.

  15. Jonathan says:

    A great explanation with the limiting space.

    My only contention would be with the word ‘happens’ in the sentence: “…and all this stuff, at various positions in space, happens over and over again…”

    Happens already seems to imply something that occurs over time. It feels like you’re taking a long period of time and breaking it up into smaller periods but that doesn’t quite get to the bottom of the question.

    Perhaps something closer to exists than happens?

  16. Tony Rz says:

    Somehow I knew that my father would soon die, how I’m not sure, but I did know. The night before his death I said to myself that I would not ever see him alive again and the next day he died in a car accident. In somewhat the same manner I knew that my mother would soon die and when I heard the phone ring I knew that she had. She died sitting in a chair during an afternoon nap.

  17. Santosh KC says:

    Time is something that keeps everything is order just like the page numbers in the book as you mentioned. If there is no time then it would be pretty bizarre that anything can happen at one ‘moment’ or nothing happens.

  18. Alan Dix says:

    In your comment Ramon, you say:

    “Time is a collection of volumes, or spaces ‘next to’ each other.

    The space now, ‘next to’ the space now, ‘next to’ the space a minute from now, and so on. ”

    This is Newtonian idea of time, what I always thing of lasagne time, layers of now on top of each other. Einstein’s great step was realising ‘now’ is a human construct not a physical phenomena; I forget his exact quote, but something like “time is what it says on your clock”, but of course depends on whose clock. This is more like spaghetti time, each of us carrying a stream of time with us, but our ‘now’s only meeting in the here-and-now when our spaghetti strand life paths actually intersect.

    … but I’m not sure whether the pasta metaphor would appeal to an 11 year old!

  19. Jim Carroll says:

    I bump into your blog occasionally and enjoy it when I do (typically getting suitably frustrated by your seeming lack of understanding of some concepts outside of your physics specialism …) but I reckon as an 11 year old I’d find this time explanation a tad condescending.

    Now I’ve stated that, a fair challenge is for me to provide a better one … I’ll get back to you!

    (Hello from a fellow Carroll)

  20. Jason says:

    I can clearly remember when I was about 11 being driven up the wall by explanations like the one you’ve given. I’d be careful to formulate my answer as clearly as I could and the response from almost everyone except my parents would be an answer modified to be “suitable for a child”. A child may have a smaller vocabulary (although that’s not always the case) however that doesn’t mean that they want or need an answer tailored to be strange, wrong or filled with zero content placeholders instead of
    information. Your sentence “Time is not hard to understand!” manages to be strange (if it wasn’t hard to understand then it wouldn’t be hard to explain in far less than 300 words), wrong (time has been the subject of speculation and enquiry for centuries and
    remains a subject of much controversy) and a zero content placeholder (the bald statement that it’s not hard to understand gives no understanding, simply tells the listener that they’re dumb for not getting it in the first place). That’s a pretty impressive count for just one sentence. “But wait there’s more!” as they say on
    latenight tv advertising. The exclamation mark is a sign that what follows will not be a carefully reasoned train of thought but an attempt to engage the reader with excitement. If as a child I asked a question it *wasn’t* because I felt a lack of excitement in my life, nor did I feel the need to be engaged in the subject. Clearly I was already interested enough to have asked the question. What I wanted was a clear explanation that contained enough context that I could fit it into the world picture that I already had, with enough detail that it made sense and would allow me to add new information to expand my world picture without ugly gaps or bits that pushed up against each other with harsh contradictions. That’s how I feel about the first sentence, thinking about what I’ve said, read
    the rest of what you’ve said or almost anything written for children, and you’ll be quickly able to see what I think of it. I feel that until we can address that problem, teaching science to children will be as hopeless as it is now. The result will be adults with the same dismal grasp on how the world works that we see all about us now.

  21. Ant (@antallan) says:

    Late to the discussion…

    Like Bob, I’d reach for Cummings – which I read as a teenager, rather older than 11, in something by Arthur C. Clarke, iirc.

    Thirty-five years on, and with a Ph.D. in theoretical physics well behind me, it still makes sense to me!

    /@

  22. I’m no scientist, and it’s past the deadline anyway, but I just came across this challenge and had to give it a shot:

    Everybody knows what time is. It’s what you find out by looking at a clock. See the clock? It’s ten o’clock. That’s the time. Time is a convenient way of organizing our lives, that’s all. So why ask, “What is time?”

    In fact, it’s one of the most difficult questions facing scientists today. Time isn’t just something we’ve come up with. It’s part of nature.

    Can you hold your breath for 30 seconds? You can’t do it forever. Nature’s put a time limit on you. Time is part of how nature works.

    What do those thirty seconds mean? Well, there had to be a start and a finish. Something had to happen. Time is a measure of change. It tells us how many things happened while something else happened. In this case, thirty seconds happened while you were holding your breath.

    What is a second? It’s the movement of a second hand on a clock. If a clock is well-made, the motion of the second hand will be reliable. We can keep time with seconds or any other regular motion. Time is how we observe the motion of the universe.

    Why is this a problem for scientists? When we are talking about time, we are talking about relationships between events, like the relationship between you holding your breath and the movement of a second hand on a clock. This relationship might seem obvious to you. Anybody can count the seconds on a clock while you are holding your breath, but science tells us that people moving at incredibly fast speeds will count it differently. It’s true. Time depends on who is watching! How can that be, if time is part of the natural order? Nature must be very weird. It is, and the science of time goes even deeper!

    299 words

  23. Carol Everhart Roper says:

    I was fascinated by this challenge, and even though I’m not a scientist, I wanted to give it a try. I like many of the submissions, but for me, I didn’t want to distill time to a single definition, but rather show kids some of the myriad ways science looks at time. So here’s my offering, 300 words.

    Time is the counting method we humans have created to document change. We all understand time and clocks. However, scientists get far more complicated when studying time. We even have a unit of time which is the shortest in which any change can be detected, called Planck Time. It’s so incredibly small that it’s impossible for us to even imagine, but it is very useful when trying to understand the nature of change.

    It can get confusing, because we also talk about things ‘outside’ of time, like the ‘spooky’ action of two particles that are ‘entangled’– change a property of one and the other simultaneously changes, no matter how far apart they are. Quantum physicists also talk about something called superposition – which means that a particle is in every possible position at all times until we try and look at it, which then forces it to stop at one position.

    Time has no fixed rate of change. Einstein figured out that space and time are two sides of the same fabric – that’s ‘relativity’, because that fabric shrinks and stretches unequally from the density of every object within it.

    You and I experience our lives at slightly different rates of time. Those differences are so small that it’s not noticeable to us on this little planet. But think of black holes; some scientists think that there’s a point inside a black hole where time stops- where no change can happen. Others think time never totally stops, but slows as the hole compacts… then perhaps explodes, creating a new universe! We just don’t know.

    My goal isn’t to give you a single definition of time, but to show you that time is about change, it’s something completely mind-blowing and understanding time will continue to fascinate us for a long, long … time.