Try this math riddle with your 6^th to 8^th grader. You have seven friends coming to your birthday party, so you need exactly eight pieces of cake. Your mom says you're only allowed to make three straight cuts, and you can't move the pieces of cake as you cut, either. How can you do it? Answer: Make an "X" across the top of the cake, using two straight cuts to form four pieces. Then, cut the cake horizontally in half, creating a top slide and a bottom slice.