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 sermon about lying
A minister told his congregation, "Next week I plan to preach about the sin of lying. To help you understand my sermon, I want you all to read Mark 17."

The following Sunday, as he prepared to deliver his sermon, the minister asked for a show of hands. He wanted to know how many had read Mark 17. Every hand went up. The minister smiled and said, "Mark has only sixteen chapters. I will now proceed with my sermon on the sin of lying."