These students in all likelihood had never, ever, been asked to produce a mathematical proof. They were being told that they were about to write one as a group, after having a general description of how to do it, without seeing any worked out examples first. It is quite possibly the first time that they had ever been expected to do anything for a first time in a math class without seeing a worked example first.

Furthermore they were staring at a theorem that was complicated and abstract enough that none of them had any idea why it was true. They had just been given a method of attack, but had no particular reason to have any confidence in it.

I expected and wanted this surprise. I wanted them to take this handout seriously. I thought that the experience of seeing how easy an apparently impossible problem really was would make them pay attention.