One of these things is not like the others.
One of these things just doesn't belong.
Can you tell me which thing is not like the others,
Before I finish my song?
-- Sesame Street
In the 9/11 attacks, UA175 hit the WTC South Tower, AA11 hit the North Tower, and AA77 hit the Pentagon.
In contrast with the twin towers, the Pentagon looks like a pancake from the air. It's no taller than the tail section of AA77. AA77 took a very roundabout way of targetting the Pentagon: skimming the ground at three times landing speed, without inadvertently touching the ground, then hitting the Pentagon precisely where it would do the least damage -- a spot still under construction, and the wing built to best withstand such an attack.
The twin tower destruction was the spectacular attack, whereas the hit on the Pentagon was the self-injury designed to make the criminal look like a victim.