From "The Practice of Programming" by Brian W Kernighan & Rob Pike:

Another effective debugging technique is to explain your code to someone else. This will often cause you to explain the bug to yourself. Sometimes it takes no more than a few sentences, followed by an embarrassed "Never mind. I see what's wrong. Sorry to bother you." This works remarkably well; you can even use non-programmers as listeners. One university computer centre kept a teddy bear near the help desk. Students with mysterious bugs were required to explain them to the bear before they could speak to a human counsellor.