In "Funny Ha-Ha, Funny Strange, and Other Reactions to Incongruity," John Morreall discusses this classic Addams cartoon as an example of a visual joke. No doubt he's right: it's funny. But it's also a great illustration of the close relationship between magic and comedy.
Imagine the cartoon without the uphill skier looking over his shoulder. No longer quite so funny? Why not? The answer, I think, is that our effort to imagine what the uphill skier might have witnessed—an effort that ends in failure—is a big part of our amusement. And it's here that there's a close relationship to magic.
Suppose that David Blaine is skiing downhill toward a large tree in untracked powder. Just as Blaine approaches the tree—which also stands in untracked snow—a simple black curtain drops and obscures his passage. A moment later, the curtain is snapped back and Blaine, still skiing, appears to have left tracks that go around the tree, just as in Addams' drawing. The result is a performance that, like Addams' cartoon, all but forces you to try to imagine: how? What would you see if you could, like the uphill skier in the cartoon, look behind the curtain?
Addams' classic cartoon is like a single-frame picture of a complete magic trick. It amuses us precisely because it leaves the mind in a specific sort of tailspin—just like good magic.