Now you're getting to the heart of the matter. One way to isolate the core of the problem, to find out how to ask the right question, is to try a few examples.

Let's label the two trashcans as Can A and Can B, and suppose the skateboard is in Can A (this assumption won't harm your reasoning. Why?)

Suppose you try the obvious and make your one question: in which can is the skateboard?

This won't work at all. If the geek you ask happens to be the liar, you'll be told Can B; otherwise, you'll be told Can A. Either way because you don't know the identity of the liar the information is useless.

And that's it, the main obstacle. You have two possibilities for the liar and only one question.

Before you go back, take a few minutes to think through a couple more examples. Try to think of a way around this obstacle.