When two men place the ends of a toilet roll tube into their bum holes and shit through it into each other's arses.
Last night I went over to Matt's and he told me to shove the end of a toilet roll tube into my arse. Once I'd done this he paced his anus on the other end and we proceeded to poo into each other. I asked him what we'd just done and he told me we'd just been commuting the Bakerloo line. We'd not even left the house.