One-to-one Correspondence :  Suppose you are given a certain number of cups and saucers, and are asked to find out whether there are enough saucers for all the cups. How would you do this? By counting the number of cups and saucers, of course ! But what if you do not know how to count? Can you still answer the question? Yes, by matching (or pairing) each cup with a saucer. This means that you set up the cups and saucers in one-to-one correspondence with each other. Thus, pairing 'is simpler than counting, and is basic to it. You may have come across a situation similar to the following one, when interacting with preschoolers who are learning how to count.