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 test for being drunk
A police officer pulls over this guy who had been weaving in and out of the lanes.

He goes up to the guy's window and says, "Sir, I need you to blow into this breathalyzer tube."

The man says, "Sorry officer I can't do that. I am an asthmatic. If I do that I'll have a really bad asthma attack."

"Okay, fine. I need you to come down to the station to give a blood sample." "I can't do that either. I am a hemophiliac. If I do that, I'll bleed to death."

"Well, then we need a urine sample."

"I'm sorry officer I can't do that either. I am also a diabetic. If I do that I'll get really low blood sugar."

"Alright then I need you to come out here and walk this white line."

"I can't do that, officer."

"Why not?"

"Because I'm too drunk to do that!"