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."

2 Mexicans
A Border Patrol agent is on duty. He spots two Mexicans and runs them down. They show him their papers (he thinks they are phony).

He tells them, "O.K. I have a test for you. I want you to use the words 'cheese' and 'liver' in a sentence."

So, the first guy says, "I made a liver and cheese sandwich for lunch."

The agent says, "That was good, you can go. What about you?" he asks the second guy.

He says, "Liver alone. Cheese mine."