What is the funniest joke you've been told that you still think about to this day?

[Answered on Quora.com by Nick Thompson]

An Irishman goes to a building site in London and asks the foreman if he can have a job.
“Are you Irish?”, asks the foreman. “Well sor, that I am”, replied the Irishman. “Well”, said the foreman,”I don’t want any more Irish on this site, because they don’t know nuffink. Last week we have one here who didn’t even know the difference between a girder and a joist. Can you tell me the difference between girder and joist?”
The Irishman thought for a little while, and then replied, “Well sor, I tink that Joyce wrote Ulysses and Goethe wrote Faust.”