Number 10

Listen up, dummy! This joke plays on the double meaning of "9/11." On one hand, it's the sequence of numbers after 9 and before 11. On the other hand, it is a dark reference to the September 11 attacks, also known as 9/11, a series of four coordinated terrorist attacks by the militant Islamist terrorist group al-Qaeda against the United States on the morning of Tuesday, September 11, 2001. So, the joke is that the number 10 is stuck in the middle of the horrific event of 9/11, causing him to have PTSD. Get it now, airhead?