Why does an object float or sink when placed in water?

When you put something in water, it either floats or sinks depending on how heavy it is and how much water it pushes aside.

Imagine you're in a bathtub. If you’re full of air, like when you’re super happy and you float around like a balloon, that’s because your body isn’t as heavy as the water you push out of the way. But if you're all squished down, like when you're sad and you sink to the bottom, that's because your body is heavier than the water it pushes aside.

Floaty things, like a toy boat or even a big ship, are usually made from light materials. They don’t push too much water out of the way, but they’re not very heavy either. It’s kind of like being on a trampoline, you don’t need to jump high to bounce around.

Sinky things, like a rock or a metal spoon, are heavy and don’t push aside much water. They're more like a backpack full of books, it's hard to float when you're carrying all that weight!

So the rule is: if something pushes aside as much water as it weighs, it floats; if it’s heavier than the water it pushes aside, it sinks!