.. how they work and when to use them.

It is also up to ourselves to understand how to use them. You are skipping over that completely. Adding enumerable properties to prototypes, esp. Object ( and Array ), is ++UnGood. And you can test for that.

Adding properties to Object.prototype just isn't 7 kyu kata material; there's too much subtlety and possile breakage involved.

My opinion.

I think you should consider removing this one. Solutions which could be considered 'correct' are really bad practice, and if you have to warn against it in the instructions it's probably not a good thing to expose students to. I spent quite a lot of time looking for other solutions becase I thought, "Surely he doesn't mean..."