You have two spheres with the same outside dimensions, the same volume, and the same mass. The outside parts are made of the same material. The difference: one is hollow inside. How can you determine which is which? (No blunt-force instruments allowed.)
SOLUTION TO BLACK CAT PROBLEM from July 3:
If a black cat crosses in front of you and then crosses back, is your bad luck doubled or canceled?
DOUBLED if the cat is a scalar; CANCELED if the cat is a vector.