Physics and astronomy activities with 4-6 year-olds

This page is now superseded by my blog, but still contains a few activities noty documented there.

This year, my son's pre-K/K class is organized around the theme of water, so I have been selecting physics activities related to water. Many of these are not original, but I tweaked them and learned enough by doing them with the kids, that I feel this advice might be useful to others.

Floating and sinking

The first activity is very simple but builds a foundation for the following activity. Here, we experiment with what floats and what sinks. Each child should get one full glass of water, and the whole table shares small pieces of many different materials: wood, styrofoam, plastic (as many different types as you can find...not ALL plastic floats!), hard-boiled eggs, chunks of metal (maybe coins), and aluminum foil. Bring spoons so you can easily retrieve things that sink.

Mostly, you can just let them experiment, but prod them to make predictions from time to time. Also let them grab stuff from the environment (leaves, seeds, etc) and try those two. But there are two things worth singling out:

(1) the boiled egg will sink in water but float in very salty water. So prepare one glass of water with enough salt, and let kids borrow it. You may also find that some types of plastic float and sink just like the hard-boiled egg.

(2) the aluminum foil floats when flat but sinks a bit when balled up. To get it to really sink, you need to squeeze amazingly hard to get all the air bubbles out. This leads naturally to the idea of submarines in the next activity. Furthermore, you can have them build boats by making a few simple folds in the foil, and they can put cargo in it and see how much cargo it can sustain.

I did this activity with eight 4-5 year-olds, in two groups of four. Most of the interest was gone after about ten minutes, but there was one in each group who remained intensely interested until we really made them leave. In each case, I think it was a younger child who was most interested. The older children kind of knew what would float, so it was nice to have the build-a-boat option to keep them interested. (The boys also liked the challenge of trying to compress the ball of aluminum foil.) Some books recommend other fluids such as cooking oil or vinegar, but while rehearsing at home, I found that pure water and salt water captured the interesting range of densities and was MUCH easier to deal with.

Building a submarine

This builds very well on the previous activity. I found some books and websites that describe the basic idea: take an empty plastic soda bottle, strap on some ballast (like a roll of quarters) so it barely floats, drill a few small holes in it so water can enter/exit, and then drill a hole in the cap and feed a straw through it. Once in the water, sucking on the straw pulls water in and makes it sink, while blowing on the straw expels the water and makes it float again.

In practice, I found the ballast unworkable. An enormous amount of ballast is required to make it almost-sink, and it easily slips off. If less ballast is used, one must suck quite a bit of water in to make it sink, and I didn't think the kids could handle that; the lag between starting to suck the water and sinking was too long. On the other hand, too much ballast would make it sink with no water intake, and that was obviously bad. So I had to fine-tune the ballast a great deal. The kids would have to precisely follow a set of fairly rigid instructions to make it work, and I wanted the kids to be more free.

So I came up with the idea of having the kids put the ballast into the bottle, stone by stone, until it was almost ready to sink. This was the most successful part of the activity! The kids were very patient (only one out of ten got discouraged at how slow it went), and they got a lot of practice making predictions that it would sink, testing those predictions, and modifying their hypotheses. It takes an enormous amount of ballast to sink this thing!

So we had an opportunity to discuss how safe ships can be, that even though they're made out of metal which by itself would sink, a modest amount of air can keep it up.

As each student got his/her bottle to nearly sink, I quickly drilled the required holes, put the straw in (I would recommend having spare straw-filled caps handy...starting with the straws in is likely to be too much of a distraction), and let them try diving and surfacing. I would recommend using a shallow container or water, NOT an aquarium! Water deeper than say 6 inches is just unnecessary and a pain. Mastering the dive/surface cycle was difficult for these kids (4-5 year olds), but a few did it, and the rest just had fun blowing bubbles through their submarines.

This activity was pretty successful in terms of student interest. Even if they didn't master the dive/surface cycle, they never got discouraged or stopped having fun, and many wanted to take their submarines home.

Observing drops of water

A drop of water takes on very different shapes depending on the underlying surface. The basic idea here is to practice observing skills rather than to teach about surface tension, though! The equipment is very simple: eyedroppers, a glass of water, aluminum foil, wax paper, graphite pencils, and pennies.

Start with as few distractions as possible. Just put one drop of water directly on the table in front of each child, and ask them to observe it. I let them (again, 4-5 year olds) each have their own eyedropper from the start, and I think this was a mistake. They emptied the whole dropper or otherwise played with it, and got the table wet. At that point, I abandoned my original idea of having them draw the drop of water! When I asked about its shape, they invariably said "circle", and it took some doing to get their eyes down at table level to look at the cross-section of the drop. Not very interesting at this point, but it sets a baseline, and I think they would have benefited from being able to draw it.

Next, the alumninum foil. The water goes amazingly flat. You can challenge them to make a taller drop of water, and they'll try, but they can't do it.

Next, the wax paper, where the drop stands up amazingly tall. I wanted the activity to be about developing powers of observation, but they asked me "why" so much that I told them how the water is made of molecules that "like" aluminum so they "spread out and hug it" but don't "like" wax paper, so they curl up together as much as possible.

Now, hand each student a pencil and ask them to see if the drop of water "likes" the tip. They have a great time making the drop follow the pencil all around.

Finally, hand out pennies and ask how many drops they can put on the penny without overflowing. At this age, more than half the kids just wanted to overflow it and have fun. That was ok, but we did also try to get them to see the shape of the water on top of a penny before it overflowed. Others went further and tried to beat the teacher, or at least the other students, in maximum number of drops without overflowing. This gave us a chance to discuss the importance of being careful when doing experiments, and practicing to improve technique.

Overall, this activity generated less interest than the submarine, but just as much as the floating/sinking activity.

Big Bang

I was asked to talk about the Big Bang to a first-grade class because they were making a timeline (like, from pioneer days to present) and wanted to know when it should start. I was reluctant, but gave it my best shot. I came up with a kinesthetic activity simulating galaxy motions best done outdoors, but first we stayed inside and discussed what galaxies are.

This actually involved a lot of ideas about sizes and distances in astronomy. We started by talking about how the Sun is a star, even though the stars appear much fainter (for the same reason that flashlights appear fainter when further away). I was able to elicit a lot of this from them, without a "lecture." We talked about how the Sun is 100 times bigger than the Earth, but then I got a globe and asked them to imagine a pile of globes 100 globes tall and 100 globes wide on all sides. That would take a million globes to fill in!

Then I had them observe parallax with their fingertips, and explained that we use that to find the distance to the nearest stars. I got a roll of toilet paper and said let's pretend that the width of a square represents the distance from the Earth to the Sun. I started to unroll the paper and asked them to yell "Stop!" when they thought I had reached the next nearest star. Of course, it would take almost 300,000 squares, so I had to say many times, "No, it's further." When the roll was all unrolled, I told them it would take over 500 rolls! (When I do this in class with more preparation, I hide a cart with 500 rolls backstage and wheel it in, and the gasps are audible.) So then we talked about how a galaxy (I had a picture ready) is full of billions of stars, each so far apart, but that is still closely packed compared to the empty space between galaxies. Then we went outside to a large grassy area.

We started in a dense clump, with each person representing a different galaxy. I told them to walk away from each other, with exaggerated walks, and to freeze when I said freeze. I counted "1....2.....3" while we walked, and then said freeze. They looked around at their friends frozen in the exaggerated walking position...this is like us seeing other galaxies; we know that they're moving away from us, even though observations are so short that we can't see them get to the other side of the park. (A parent asked, "How DO we know how fast they are moving?" I gave the analogy with police radar.)

And because we're all moving away from each other, we can tell that in the past we were closer together. Ask them how long ago it was that we were all crowded together on top of each other. Hopefully someone remembers "3". Rephrase it: at the speed that you were walking, it would take 3 seconds to go from all-together to the spread-out positions you are now. Similarly, at the speed galaxies are moving, it would take 14 billion years to go from all-together to the spread-out positions they are now.

I then talked about direct evidence that we were all closer together in the past. I asked them what it feels like when all crowded together; it took some work to get the response I wanted: "hot". I then asked, if they saw a lump of dough and a loaf of bread, could they tell which one had been in a hot oven? Similarly, we can tell that the materials around us have been cooked (Big Bang nucleosynthesis) a long time ago, during the time when everything was crowded together and hot.

I was not able to think of a great way to explain what physicists mean by "time began" with the Big Bang. So I discussed extrapolation and its limits, using their own growth as an example. If they're growing an inch per year, do you think in 50 years they'll be 50 inches taller? No! Similarly, in the past we can extrapolate their body size to a moment when they were zero size, but not before that. There was really a moment when their biological clock started. Similarly with the universe and the Big Bang.

Disclaimer: I know there are vast simplifications here. These are first graders! But if you have better ideas on reaching first graders with these concepts, I'd love to hear them.