I had no idea until after Travis and I made this little project that this is a classic experiment dating back to… the 1600s! It’s so-named because it was first described in detail by Rene Descartes, and it’s a neat way to show kids some basic principles of pressure and density.

First, we made our little diver – and by little, I mean little! Measure a 1 inch x 1.5 inch rectangle on aluminum foil (we folded our foil double so it was a little thicker and sturdier).

Draw a little diver shape onto this rectangle and cut out.

Travis thought he was so cute!

Cut 1.5 inches from a bendy straw. Bend into an upside-down U and attach to your diver with a small paper clip. Be sure to use a small one! Our first was big and so heavy our diver sank right down.

Now add a tiny bit of clay to his feet. Test that he doesn’t sink in a glass of water before you use a bottle; if so, just remove some clay.

If he’s horizontal, you need a little more clay. We made the mistake of skipping this step on our first try, although Travis thought it was hilarious to fish our diver out from the bottle once we realized he was sinking.

Once we had the proper diver who floated just right, we filled an empty 2 liter bottle with water nearly to the top. Screw the cap on tightly.

Squeeze the bottle and watch him go up and down! He’ll sink when his straw fills with water, increasing the density just enough, then float when you release the bottle and the water drains out of the straw again.

Ours wasn’t as perfect as some example we saw online, but we had fun!