Mary, if buildings were still built of wood and stone only, they wouldn't be taller than six stories. The twin towers were actually a marvel of engineering, and were built of steel and concrete. They were built to withstand earthquakes and tornadoes and hurricanes. Who knew about jetliners? I won't end on a glib note though--I recall in documentaries that planes were considered, but I don't remember the conclusions reached. The only other plane/skyscraper incident I remember is a small plane lodging itself in the Empire State Building back in the 1940s, and though the occupants of the plane died, the fire was put out and the building repaired without too much difficulty.

SRS