Red bumper sticker, reading "If this sticker is blue, you're driving too fast."
An experimental physicist walks into a theorist's office and hands him a graph, wondering if he can explain the results. The theorist stares at it for a long time, then says confidently, "Well, I see that you have a peak here. This is precisely where you'd expect to have a peak because..." and goes on a lengthy and incomprehensible rant. After about ten minutes, he suddenly stops and frowns for a moment. Nonchalantly, he turns the graph around and starts anew: "Well, you have a trough here. This is precisely where you'd expect to have a trough because..."
Another bumper sticker: "Physicists Against Drunken Physics: Don't Drink and Derive."
A biologist, a physicist and a mathematician are visiting a foreign country. As they leave the airport in their car, they pass by a field, in which stands a single, black cow.

The biologist says, "How interesting. The cows in this country are black."

The physicist says, "You can't say that with certainty. All you can say is that they have cows in this country, and that at least some of them are black."

The mathematician says, "You can't say that. All you can say is that there is at least one cow in this country, and that at least one side of it is black."

A farmer wants to hire a brainy type to construct a new farm for him. He selects a physicist and a mathematician and decides to test them. He gives each a hundred meters of fencing and asks them to construct a fence that will enclose the maximum possible area.

The physicist decides that this is easy, whips out his calculator, figures out the radius of a circle with 100m circumference and builds the fence, confident that he cannot be beaten.

Everyone watches in amazement as the mathematician snips off about a meter of fencing and builds a tiny little fence around himself. He then beams triumphantly and announces, "I declare myself to be on the outside of the fence."