This classic family riddle sounds impossible at first: two mothers and two daughters ate only three cookies, yet each person ate exactly one. How can four people share only three cookies?
The answer is that there are only three people: a grandmother, her daughter, and her granddaughter.
The grandmother is one mother. Her daughter is both a daughter and a mother. The granddaughter is the second daughter. That gives us two mothers and two daughters, but only three people total.
Each of the three people can therefore eat one cookie, using exactly three cookies. The trick is that the family roles overlap. One person can be both someone’s daughter and someone else’s mother at the same time. Once you notice that, the puzzle becomes simple.
