Sink or Float?

Part One · Guess, Then Test

Will it sink, or will it float? Here is a game for a kitchen table. Fill a big bowl with water, and pick something to drop in. But wait, because you must guess first! Detectives always guess, then they test, and then they write down what happened.

Start with an ice cube. Guess first, then drop it in, and it floats! Ice is made of the same water, but its pieces are spread out a little more. Scientists say that ice is less dense than liquid water, so it stays on top.

Next, try an orange that is still in its peel. It feels heavy in your hand, so you might guess that it will sink. Plop, and it floats! Even though it feels heavy, something about the orange is light, and the next page tells you what.

Here is the clue. An orange's peel is porous, which means it is full of tiny pockets of air. Air is very light, so the peel makes the whole orange less dense than water. It works like a puffy life jacket for the fruit inside it.

Now peel the orange, and drop in only the fruit. Guess first! Splash, and down it goes, because without the airy peel, the fruit is more dense than water. It is the very same orange, and yet it floats in one coat and sinks in the other.

A lemon is trickier. A whole lemon floats, but with the peel off, some lemons float and some sink. A lemon is very close to water in density, so small things can tip the answer. Try yours, and write down what you see.

Part Two · The Clay Puzzle

Now for a puzzle. Roll a ball of modeling clay and drop it into the water. Glug, glug, and it sinks, because clay is denser than water. So how could anything made of clay ever float? Use oil-based modeling clay for this, because play dough goes soft in water.

Take the very same clay and press it into a boat. Make it wide and flat, with high sides like a bowl, and set it gently on the water. It floats! You did not add any clay, and you did not take any away, because you only changed the shape.

Here is the reason. The little ball pushes aside only a small bit of water. The wide boat pushes aside a great deal more. The more water something pushes aside, the harder the water pushes back up. The boat shape pushes aside enough water to hold up all of the clay.

So, will it sink or float? Now you have two clues to check. How dense is it, compared with the water? And what shape is it? Then drop it in the bowl, and watch what happens. Pick something from the kitchen, and guess first!