What's up with the ''pie in face'' gag?

The eternal gag you keep seeing in cartoons, where someone recieves a pie in the face. What is the origin of this? When did this first happen? Why is it so classic? Have you ever seen this happen for real?
Who has money to spend on wasting perfectly good pies this way? What if it came right out of the oven before it was thrown? The person getting it would be injured by getting burned!