Let us consider a quiz question whose answer is a proof. For instance, a question in mathematics, such as prove that for every natural number $n$ the quantity $n^3-n$ is even.
How could such a question be implemented in a computer interface so that the computer can check the proof for correctness?
A simple, yet unreasonable way would be to have a multiple choice format for the question, where every choice was one way of proving the relationship, but I guess the shortcomings of this are obvious...