This one's for all of the conspiracy theorists out there. At the link, you can watch videos of this test performed in 1988, but here are the results.
And just to make sure I'm clear - there are many who still believe that the plane that crashed into the Pentagon on 9/11 wasn't a plane at all. This is because they cite that there wasn't any debris left. Well, these videos and tests from 1988 prove that if a plane crashes into a concrete object that no debris will be left. So if for some reason you still don't believe it was a plane, then there's nothing else I can say to you...
Here's some info from the Sandia National Labs site:
The purpose of the test was to determine the impact force, versus time, due to the impact, of a complete F-4 Phantom — including both engines — onto a massive, essentially rigid reinforced concrete target (3.66 meters thick). Previous tests used F-4 engines at similar speeds. The test was not intended to demonstrate the performance (survivability) of any particular type of concrete structure to aircraft impact. The impact occurred at the nominal velocity of 215 meters per second (about 480 mph). The mass of the jet fuel was simulated by water; the effects of fire following such a collision was not a part of the test. The test established that the major impact force was from the engines. The test was performed by Sandia National Laboratories under terms of a contract with the Muto Institute of Structural Mechanics, Inc., of Tokyo.