Debugging or solving a problem by explaining the problem out loud to either yourself, a colleague, or even a rubber duck.
Repeating the problem to a third person forces you to think about the problem so you can desribe it adequately. Often this act of thinking about it in a subtley different way will help you solve the problem on your own in a "EUREKA" (or more often than not "oh, DURRH, of course!) type moment.