A student's science joke

[Dr. Strange]

25. This one is knot funny:

A woman walks into a bar accompanied by a dog and a cow. The bartender says, "Hey, no animals are allowed in here!"

The woman replies, "These are very special animals."

"How so?"

"They're knot theorists."

The bartender raises his eyebrows and says, "I've met a number of knot theorists who I thought were animals, but never an animal that was a knot theorist."

"Well, I'll prove it to you. Ask them them anything you like."

So the bartender asks the dog, "Name a knot invariant."

"Arf, arf" barks the dog.

The bartender scowls and turns to the cow asking, "Name a topological invariant

The cow says, "Mu, mu."

At this point the bartender turns to the woman, says, "Just what are you trying to pull" and throws them out of the bar.

Outside, the dog turns to the woman and asks, "I should have gone with the Jones polynomial shouldn't I?"