The longer answer
How heavy something is for its size has a name: density. A brick and a sponge can be the same size, but the brick is packed much more tightly, so it is denser and much heavier.
When you put something in water, it pushes water out of the way, and the water pushes back up. Things that are less dense than water get pushed up more than they weigh down, so they bob at the top.
An orange floats because its thick peel is full of tiny pockets of air, which make the whole orange light for its size. Peel it, and the fruit inside is denser than water, so it sinks.
Three things to know
- A huge ship made of heavy metal can still float. Its hull is shaped like a wide hollow bowl full of air, so the whole ship is light for the amount of water it pushes aside.
- Oil floats on top of water. Oil is lighter for its size than water, so it gathers in a shiny layer on top instead of mixing in.
- Salty ocean water holds you up better than a swimming pool. Salt makes water a little heavier for its size, so it pushes up harder on anything floating in it.
Try it at home: a sink or float test
- Fill a big bowl or the sink with water.
- Gather small things: an orange, a coin, a cork, an apple, a spoon, a toy.
- Before each one goes in, guess: sink or float?
- Try the orange with its peel on, then peel it and try again.
Ask whether the floaters are all light, or all big. The answer is neither: they are light for their size.
Quick quiz
Watch the episode first, then tap an answer.
My orange floats on the water like a little boat. Why does it do that?
Watch me push the orange right under. What does it do?
Then I peeled my orange and put it back in. What happened?
You got 0 of 3. The app has all 5 questions read aloud, then a certificate, a coloring page and a jigsaw. Get the app
