You're trapped in a room with nothing except one match, and three identical ropes in it. Let's say that you want to know when 45 minutes has passed so you can call your non-existent girlfriend on your non-existent phone. For the point of the puzzle, we're also going to assume that you're incapable of counting the time yourself, that you're wearing a balerina costume, and are a raging alcoholic just for fun.
So anyway, the only knowledge that you posess of the ropes is that it always takes 30 minutes to burn one. But you don't know if the ropes will burn at an even rate. How do you determine when 45 minutes has passed?