WHAT MAKES THINGS FLOAT AND SINK?
Quick answer: Whether or not something floats or sinks in water has to do with weight and displacement. An object will sink into water until the weight of water displaced (which can be affected by the shape of an object) is equal to the weight of the object. If the water weight that is displaced is less than the object’s total weight, the object sinks.
Check out the Sea Monkey & Bob Teacher's Guide for related activities, including this print-ready activity sheet:
Here are some videos you might find interesting:
What causes objects to float or sink? - on Reference.com
How Things Float - On Seaperch.org