We used a bunch of toys and threw them in a sink full of water to see what would happen to each.
We talked about why, this one sunk because it had holes in it, and see how it lets the water in? And this one floats because it is full of air and is has a lower density than the water.
He really had a fun time and after just a few times, I was surprised to see how accurate his hypothesizing became on whether or not a particular toy would sink or float.
"I sink in deep mire, where there is no standing: I am come into deep waters, where the floods overflow me."
Hunter is 2 years, 5 months old