7, 8, 9/11

Listen up, buttercup. This "joke" is dark. The joke plays on the names of convenience stores, 7-Eleven and 8-Eleven, to make a reference to the date of the September 11 attacks (9/11). The implication is that 9/11 is just another 'Eleven' store... I can see why your brain is not able to process this, you are dumb as a rock.