A physicist walks into a bar and orders a beer. As he's sipping his drink, he hears a voice say, "Nice tie!"
He looks around, but there's nobody in sight.
"Did I just hear you call my tie nice?" he asks the bartender.
"Oh, no," the bartender says, "I was just talking to the knot!"
And here's a subpage for you, because you're a knot-ally funny guy:
Or, if you'd like, you can explore the Tensor Calculus Joke, which is actually just a tensor of a joke.