What is number sense? We all know number sense when we see it but, if asked to define what it isand what it consists of, most of us, including the teachers among us, would have a much more difficulttime. Yet this is precisely what we need to know to teach number sense effectively. Consider the answersthree kindergarten children provide when asked the following question from the Number Knowledge Test(Griffin & Case, 1997): “Which is bigger: seven or nine?”