A horse walks into a bar, and the barman says, "
Why the long face?"
A bear walks into a bar, and says, "
I'd like a...
...pint of lager please" and the barman says, "
Why the big pause?"
William Shakespeare walks in to a bar, and the landlord says, "
Get out, you're barred!"
Rene Descartes walks into a bar and orders a drink. When he's finished, the barman asks, "
Would you like another?" Descartes replies, "
I think not" and disappears.
Not the best jokes in the world (and the middle two might take a while to get until you say them out loud), but I've always loved the way they follow a theme and become progressively more clever.