Squares are worth knowing because a^2 - b^2 = (a+b) (a-b). This means any two numbers which are close together can be multiplied by finding the square of the midpoint.<p>For example, 79*71 = 75^2-4^2 = 5625 - 16 = 5609.<p>If you know your squares up to 100 and your multiplication tables up to 20, you can solve a great deal of two digit multiplication problems without having to reach for a calculator.