This guy goes into a bar and asks for a pint of beer.

This guy goes into a bar and asks for a pint of beer. While the barman's
pouring it, he looks about the place, and sees a piano man sitting there in
front of the old ivories, giving it a bit of a play, and his pet monkey, which
is wandering about with a tin mug collecting money. Now just as the barman
finishes pouring the guy's pint, and the guy pays him, the monkey drops its
tin cup, charges along across the floor, up onto the bar, unzips its flies,
and dunks its dick into the guy's pint. At which, the guy turns to the piano
man and says;
"Do you know your monkey's dipped its dick in my beer?"
To which the piano man replies;
"No, but if you hum it, I'll improvise!"