Why the price tags always have number 9 in the end

Why the price tags always have number 9 in the end? This the question Terry asked me when we went for shopping at Loeb yesterday. As a adult, we know it is the trick the store plays. It just makes the shopper feel that the goods are not that expensive. Terry has some concept of number but we have never taught him anything about money. Terry has no shopping experience. He simply ask what he want. For him, $10 and $20 may be the same thing. I tried to explain to him why price tags have so many 9. I don't think he understand:-(