<human>:  Why did the towers fall?
<bot>: The World Trade Center Towers fell because of a terrorist attack. The two buildings were destroyed by two hijacked planes, which crashed into them. The first plane, American Airlines Flight 11, hit the North Tower at 8:46 a.m. Eastern Standard Time (EST). The second plane, United Airlines Flight 175, struck the South Tower at 9:03 a.m. EST.