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.

(In reply to No Subject by armando)

Armando: If you have a spare cat, try the following experiment:
Butter one slice of bread
Strap it on the cats back, butter facing outwards
Take the cats paws and let fall

Theory: The bread will try to fall on the buttered side while the cat wants to fall on its feet.

Result: If you put the right amount of butter on the bread, the bread/cat should hover above the ground. By putting more butter or scraping off butter, you can influence the cats height.

There are a number of related, interesting, experiments if you have four theoretical cats available. 

Posted by Hugo on 2005-03-25 19:29:24