A lecture about English
A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."

A tourist wanders into a back-alley antique shop...
A tourist wanders into a back-alley antique shop in San Francisco's Chinatown. Picking through the objects on display he discovers a detailed bronze sculpture of a rat. The sculpture is so interesting and unique that he picks it up and asks the shop owner the price. "Twelve dollars for the rat, sir," says the shop owner, "and an extra thousand for the story behind it." "At that price, you can keep the story, old man," he replies, "but I'll take the bronze rat." The transaction complete, the tourist leaves the store with the bronze rat under his arm. As he crosses the street in front of the store, two live rats emerge from a sewer drain and fall into step behind him. Nervously looking over his shoulder, he begins to walk faster, but every time he passes another sewer, more rats come out and follow him. By the time he's walked two blocks, at least a hundred rats are at his heels, and people begin to point and shout. He walks even faster, and soon breaks into a trot as multitudes of rats swarm from sewers, basements, vacant lots, and abandoned cars... following him. Rats by the thousands are at his heels, and as he sees the waterfront at the bottom of the hill he panics and starts to run full tilt. No matter how fast he runs, the rats keep up, squealing hideously now not just thousands but millions, so that by the time he comes racing to the water's edge a trail of rats twelve blocks long is behind him. Making a mighty leap, he jumps up onto a lamp post, grasping it with one arm, while he hurls the bronze rat into San Francisco Bay as far as he can throw it. Pulling his legs up and clinging to the post, he watches in amazement as the seething tide of rats surges over the breakwater into the sea, where they drown. Shaken and mumbling, he makes his way back to the antique shop. "Ah sir, you've come back for the story," says the owner. "No," says the tourist, "I was just hoping you had a bronze sculpture of a lawyer "