This is Bob. Drop Bob in a bottle of water. Squeeze the capped bottle and Bob will take a dive. Release, and he'll rise.

Why? See, Bob has gas.
You squeeze the bottle, Bob's bubble shrinks and he sinks.
Let go, the bubble grows, and up he goes.

Warning: Adult supervision required! Choking hazard for thirsty kids.
Don't drink Bob's bath water either. eww.

Bob is a fun toy, but he's a STEM tool too. See the STEM section below for ideas and questions

Cartesian divers are an old idea. See Wikipedia for more detail.