One of the 9/11 conspiracy theories that some people believe, is that the Pentagon was hit by a missile, and not a Boring 757.
Most people who do believe this, believe a missile must have hit, because they believe that with not much piloting training, a person could not actually fly a jumbo jet into the side of a building that's only a few stories high, and that the damage to the building doesn't appear to them as the type of damage that jumbo jet would do.
First, it's actually not as difficult to learn how to fly a plane as many people might believe. In fact many people say that learning how to fly a plane is like learning how to drive a car.
Also, you have to remember that the Pentagon is an old concrete and steel building, and that buildings from it's time period were built stronger then they are today.
A section of the outer ring of the building is the only part of the building that completely collapsed, and the shape of the section that collapsed is like that of a large object plowing into it.
A missile would not have caused the type of damage done to the building. A missile would have either have caused less damage, or the damage would have been more severe. Plus a missile would not have caused the fires to the sides of the collapsed area, as burning jet fuel from the fuel tanks in the wings of a jumbo jet would have.
Another argument is that the building should have had more damage done to it because of the wings. Again, you have to remember the building is built stronger then many modern buildings, and that the wings are not the strongest parts of a plane, plus the wings are where the fuel tanks are. When the jet hit the building, the fuel tanks exploded, causing the areas to the sides of where the fuselage impacted to start burning.
Beside the fact that the damage caused to the Pentagon could only have been caused by the impact of a jumbo jet, and not by a missile, it also makes no sense that the military would purposely attack it's most important military facility, and in the process, possibly hinder it's ability to fight a war. The conspiracy theorists also ignore the fact that every person who witnessed the impact, says it was a jumbo jet that hit the building, and not a missile, and that plane wreckage was found in the impact area.