TAKING THE BISCUIT
Tom had been meddling with his mother’s biscuit tins while she was out. One of the tins contained only Digestive biscuits, another contained only Rich Tea biscuits, and the third contained a mixture. He had carefully taken off all the labels and replaced them so that each label was on the wrong tin.
When his mother returned, Tom confessed what he had done. She reached into one tin and removed a single biscuit. From that she could deduce exactly where all the labels ought to go. Which tin did she open?