I have to make up a test for our conference math competition. Every year I choose the brain teaser category because I love them. I usually dig in books, and I have a few for this year, but I'd also like to hear your favorite ones. I include ones that make you groan such as:

How much dirt is in a hole 3ft by 4 ft by 2 ft deep?

Answer: none. There is no dirt in a hole

And complicated math ones,
Diophantus's youth lasted 1/6 of his life. He had his first beard in the next 1/12 of his life. At the end of the following 1/7 of his life Diophantus got married. Five years from then his son was born. His son lived exactly 1/2 of diophantus's life. Diophantus died 4 years after the death of his son. How long did Diophantus live?

answer: 84 years.

Anyone have some favorites?