Got this one from Braingle. This is an easy one but I thought it was fun so...
A man is trapped in a room. The room has only two possible exits: two doors. Through the first door there is a room constructed from magnifying glass. The blazing hot sun instantly fries anything or anyone that enters. Through the second door there is a fire-breathing dragon. How does the man escape?
He waits until night time and then goes through the first door