from wikipedia
"Yousef removed an altered Casio digital watch from his wrist to be used as a timer, unpacked the remaining materials from his dopp kit, and assembled his bomb. He set the timer for four hours later, which was approximately the time at which the plane would be far out over the ocean en route to Tokyo, put the entire bomb back into his dopp kit, and returned to his current seat.
After asking a flight attendant for permission to move to seat 26K, saying he could get a better view from that seat, Yousef moved to that seat and tucked the assembled bomb into the life vest pocket under that seat."
I wouldn't be surprised if terrorists got their idea from wiki(or some other Internet source), this sounds very spooky...