OK - a puzzle of sorts.
You are presented with 2 envelopes and told that one envelope contains twice as many dollar bills as the other. You are allowed to keep one envelope only.
You pick an envelope, open it, and see that it contains $8.
You are allowed to switch envelopes. Should you?
This problem is more fascinating than it initially looks. If you want to peek at some possible solutions that could end up driving you crazy then see page 2 of MIT Lecture notes on mathematical proofs