Reason: Can 0 be called a single digit? No, because there is a rule in the notation that the highest digit of a number cannot be 0. Why is it so stipulated? Because without this provision, 0 is a number, from which it can be concluded that the smallest two-digit number is 00 and the smallest three-digit number is 000, which is obviously wrong. Moreover, if there is no such regulation, it is impossible to determine how many digits a number is. For example, 15 is a two-digit number, and "0 15" becomes a three-digit number, "008". We can call the same number several numbers at will, so the concept of "number" is redundant. So the highest digit of a number cannot be "0". In other words, the smallest number is 1, not 0.