A coin collector had three pennies that looked exactly the same. However, one was a fake and weighed less than the other two. While she was looking at them, she got confused and forgot which coin was the fake.
The only scales she had were a pair of balance scales with two trays. The machine can help determine which tray is heavier, or if they balance. The coin collector thought for a while and realised she could work out the fake with only one weighing.
How did she find the fake?
The collector put one coin on each tray. The lighter coin is the fake. If the coins weighed equal amounts, they were both real coins and the fake was the one she didn’t weigh.