People seem to be forgetting that even though steel doesn't melt until it reaches around 1400 degrees celsius, it does start to weaken significiantly at lower temperatures. Jet fuel has been known to burn at anywhere between 250 and 1000 degrees celsius - in extreme cases that's certainly enough to cause a building's steel structural supports to lose their integrity and start to buckle, and that's even if you don't take into account the effects of a heavy, fast-moving, object crashing into its side. The towers were probably designed to be aircraft proof, yes, but it's not like that theory could be tested. Also keep in mind the advancements that have been made in aviation since the WTC was originally built. We're talking about different eras here.