Popular Mechanics debunks some of the popular conspiracy theories about 9/11. One example from Austin Bay's commentary:

Here's the conspiracy theory's hook: The hole in the Pentagon was smaller than the plane's wingspan. The anti-American conspirator's conclusion: voila, an American missile.

PM's experts point out the obvious: "A crashing jet doesn't punch a cartoon-like outline of itself into a reinforced concrete building." As the jet crashed, "one wing hit the ground; the other was sheared off by the force of the impact with the Pentagon's load-bearing columns. ...What was left of the plane flowed into the structure in a state closer to a liquid than a solid mass."

(Via Instapundit.)