Before you are two balls, one solid and one hollow. They are to all appearences completely identical: same size, same weight, same outer material (though one might assume, correctly, that the hollow ball would need a higher-density material on the inside to make it the same weight).
Without breaking either of the balls, how can you easily determine which is hollow?
Assume that the material is solid enough that a hitting the side of the hollow ball will not result in any noticeable echo or vibrations.
Drop both of them into a dense liquid, say tar, for example. If they both float, find a less dense liquid. Keep doing so until one ball floats and one doesn't. the floater is the hollow ball.
Solution
