The September 11 attacks, also known as 9/11, was a terrorist attack which occurred in New York City on September 11, 2001, in which multiple Islamic fundamentalist terrorists hijacked several planes and crashed them into several civilian and government settlements.
The 9/11 Challenge is a trend in which participants in a game show reenact the events of 9/11. This trend is typically only performed when two participants of the show have scores of 9 and 11. Specifically, the person with a score of 9 has to raise both of their arms to represent the towers. The Hivemind hosts have participated in the 9/11 challenge on many occasions.