It is simple. Two guys walked into the mess hall, and one asks "Hey man, where's the can in here, I gotta go". The other replies, "It's over there next to the stairway. But you need to remember to flush twice, it's a long way to the kitchen"
Just try a google search with "flush twice it's a long way" as the search term. You'll ultimately find a lot of variations. I was 14 the first time i heard it, associated with the high school cafeteria, but my father swore that it started during the Korean war. But a friend of his, who was a WW2 vet said no it was old when he joined the service. Since he outranked my father, I figured, yep I'll take that option.