From Mark Berner:
21 Cubes - a puzzle teaching kids the basic arithmetic operations. Puzzle contains 21 levels with the growing complexity. At each level, it is necessary to build a pyramid from 10 cubes. Position of each cube is a result of calculation of the two bottom ones. Starting levels are easy and have six unclosed cubes. Every next level adds more complexity and reduces provided tips. Upper levels expect logic thinking and just basic arithmetic skills are not enough here. Goal is to build big pyramid from 21 cubes. Each level adds new cube to the big pyramid.