Here's a riddle-ish physics problem I came up with: Picture a room with a refrigerator in it. The refrigerator is running and its doors are open. There is a constant supply of energy to the fridge from outside the room. Other than that, nothing enters or leaves the room. What happens to the average temperature of the room over time? Not really a riddle, but a pretty cool example of one of the first things you learn when studying physics.