I've heard 91 called the "smallest non-obvious composite". Assume that testing for divisiblity by 2, 3, 5, and 11 is easy - for 2 and 5 you just look at the last digit, for 3 you take the sum of the digits, and with two-digit numbers a number is divisible by 11 if and only if both its digits are the same. Also assume that you can recognize squares. Then 91 = 7*13 is the first number that you won't recognize as composite using these test.