There are three boxes of different sizes with three cupcakes inside each of the boxes in Mandy’s kitchen. At night her daughter wakes up and goes to the kitchen. She opens a box and eat all the three cakes.

But in the morning Mandy finds out that each box still had three cupcakes. How?

Mandy's daughter ate cupcakes from the largest box and then she kept a one of the smaller box in the empty box. In this way, each box still had three cupcakes when Mandy opened the boxes.