I have been making hard-boiled eggs every week now for the last month, and my wife has on two occasions taken uncooked eggs to work, so she asked me how to tell which ones are hard-boiled. I took out a raw egg and a hard-boiled one, put them on the counter, and spun both of them -- the hard-boiled egg will spin and the raw egg won't. Now she knows.
I learned that one from Alton on an ICA episode.
Anyone else have any stupid food tricks?