For example, with the aid of a good calculator, or even just pencil and paper, it is straightforward to multiply four or five three-digit prime numbers together. This is the mathematical equivalent of falling through the trap-door. It is easy to do.

It is quite a different matter, however, to give the huge number resulting from that muliplication to a friend and ask that person to tell you what the numbers were that you started with. That is the mathematical equivalent to un-falling though the trap-door. It is very hard to do.

The Ice Cream Stands problem, or the problem of finding the minimum dominating set in a graph is an example of a one-way function.