Then of course, there's always this old groaner:

An Irishman walks into a bar in Dublin, orders three pints of Guinness and sits in the back of the room, drinking a sip out of each one in turn. When he finishes them, he comes back to the bar and orders three more. The bartender asks him, "You know, a pint goes flat after I draw it; it would taste better if you bought one at a time." The Irishman replies, "Well, you see, I have two brothers. One is in America, the other in Australia, and I'm here in Dublin. When we all left home, we promised that we'd drink this way to remember the days when we drank together." The bartender admits that this is a nice custom, and leaves it there.

The Irishman becomes a regular in the bar, and always drinks the same way: He orders three pints and drinks them in turn. One day, he comes in an orders two pints. All the other regulars notice and fall silent. When he comes back to the bar for the second round, the bartender says, "I don't want to intrude on your grief, but I wanted to offer my condolences on your great loss." The Irishman looks confused for a moment, then a light dawns in his eye and he laughs. "Oh, no," he says, "everyone's fine. I'm just off the liquor."