Paddy and Mick are walking home after a night on the piss. They've got no money to get a taxi and are staggering all over the place when they find themselves outside the bus depot.
Paddy has a brainwave and says to Mick, "Get in there and steal a bus so we can drive home and I'll stay out here and look out for the police".
Mick duly breaks into the garage and is gone for twenty minutes while Paddy is wondering what the hell he's doing. Eventually Paddy sticks his head around the door and sees Mick running from bus to bus and looking very worried. "What the hell are you doing Mick, get a move on!"
To which Mick replies, "I can't find a number 7 anywhere, Paddy".
Whereupon Paddy, holding his hands to his head in disbelief, shouts, "You idiot Mick, steal a number 9 and we'll get off at the roundabout and walk the rest of the way!".