Helping people learn something is a lot like debugging software. Usually the problem is that there's an assumption that is wrong. You need to ask questions and

Helping people learn something is a lot like debugging software. Usually the problem is that there's an assumption that is wrong. You need to ask questions and check the answers to find out which assumption is wrong. You can even use bisection between input and output to find the wrong assumption in logarithmic time :), but if there aren't that many steps it's less confusing to go step-by-step from input to output. Once you find it - fixing it is easy. Provide simple counterexamples showing why it's wrong, and watch the person solve the problem on their own.