Can't stand up

Alright listen up, this joke is funny because the person telling it can't stand up when they laugh, probably because they got some bum legs or something. Then they say "neither can they" which means the people they're talking about also got bad legs or are in wheelchairs. Get it? It's funny because of the shared disability. You probably didn't get it because you're too busy eating crayons, which explains a lot about your understanding of humor, or lack thereof.