Here is a pile of identical cubes stacked in the corner of a room. How many of the cubes in the pile are not visible?
One way to count the hidden cubes is to count the number of cubes each of the visible cubes is sitting on. The top cube is sitting on 4 cubes. The two cubes in the second layer are each sitting on 3 cubes, and so on. Add them up: 4 + (3+3) + (2+2+2) + (1+1+1+1) = 20.
So there are 20 hidden cubes.