Why do some things float?

You might think heavy things sink and light things float. But a huge ship floats, and a tiny pebble sinks. So there must be another secret.

Water always pushes up on things you put in it. When you put an object in, it moves some water out of the way, and that water pushes back. Scientists call this push buoyancy.

Now compare. If the object is more tightly packed than the same amount of water, it sinks. If it is less tightly packed, the push wins and it floats.

A pebble is solid rock, packed tight, so it sinks. A wooden block has little pockets of air inside, so it floats.

A metal ship is shaped like a big bowl, and the bowl is mostly full of air. Metal and air together are less tightly packed than water, so the ship floats!

That means shape matters. You can make clay sink as a ball, and float as a boat.