Rohit folds a square sheet of paper in half from top to bottom, and then folds the result in half again from left to right. Holding the small folded packet, he punches two separate holes straight through it with a paper punch. When he opens the sheet out flat, how many holes does it show?

Each fold doubles the thickness, so two folds give $2\times 2=4$ layers. A punch goes through every layer, so one punch makes 4 holes and two punches make $4\times 2=8$. The traps: 2 counts the punches themselves and forgets the layers; 4 counts the layers and so answers for a single punch only; 6 adds the layers and the punches (4 + 2) where they should be multiplied.