Turtle story

One of my dad's jokes:

Three turtles is sitting in a bar. Having a great time, but just after they get the second round, it starts to rain outside.

Joe Turtle says to Ira Turtle and John Turtle: "Durn. One of us should go home to fetch our umbrellas."

Ira agrees. He says: "John, you up for it? We'll owe you one."

John says: "Okay, I'll do it. But don't drink my beer, guys!"

Time passes. And passes. And passes. Still no sign of John.

Finally Joe says: "This is getting ridiculous. Let's drink his beer anyway."

Voice from the door: "Guys, if you drink my beer I'm not fetching those umbrellas!"