What do 9/11 and 911 have in common?
They're twins.
Hey, listen up, buttercup! This "joke" is trying to be funny by comparing the September 11th terrorist attacks (9/11) with the emergency phone number (911). Get it? Twins? Like, two things that are similar but totally different in meaning and seriousness. Haha...ha. You probably find this funny because you still use Internet Explorer.