Although airplanes are constructed out of aluminum, it's a lot thicker and heavier than that can of coke on your desk.

Furthermore, the outer walls of the towers was fairly thin, and mostly made of glass.

The airliner was fully loaded, and going at top speed. I'm sorry, what did you expect to happen? For the airliner to harmlessly bounce off the tower?

After crashing through the wall and floors of the tower, the jetliner exploded, spreading burning jet fuel everywhere. It's believed that the debris from the explosion damaged the fireproof coating on the steel supports holding up the floors where the crash occurred. This is why the heat from the burning fuel was able to weaken - not melt! - the beams to the point until they bent under the weight, causing the floor to fall onto the floor beneath it. Even if that floor was entirely undamaged, it was not designed to have the upper part of the building collapsing on top of it. The force caused the beams off below to be sheared off, causing that floor to fall onto the one below it. At this point you have an unstoppable domino effect happening.