Here is a topic that he kept me wasting (?) many hours: there is a theorem that every embedding in 3-space of $K_7$ , the complete graph on 7 vertices, contains a knot. Draw such an embedding and find the knot(s)!
Every embedding of $K_6$ contains a link, but that is usually much easier to find.
If I can find the reference, I'll add that as a comment.