A stick figure is standing holding a compass (the kind for drawing circles) and a ruler, looking sad. Next to him is another stick figure, which, unlike the first stick figure, is made from perfectly straight line segments and has a perfectly circular head.
Caption: I learned in high school what geometers discovered long ago: Using only a compass and straightedge, it's impossible to construct friends. Unless you're an XKCD character, that is.
Title text: Of course, if you want the head's circumference to equal the body's length, then you can't do it. But otherwise it's completely doable. (Oh, and you need a pencil or pen as well.)