looking at the hcf (sometimes called the greatest common divisor) of two numbers
student: "Why didn't you tell us that you can just subtract the two numbers to get the hcf?"
teacher: "Because it doesn't always work"
student: "It does on the two examples we've looked at"
teacher: "Oh, so how about 5 and 15?"
teacher: "It didn't work"
student: "Well when does it work then?"