As the teacher lectures the class explaining not to use double negatives and uses various examples, the students begin to drift off in that somnolent way they have when their eyes glaze over but still continue to look in the teacher's direction. The teacher senses this and writes some examples on the chalkboard to try and see if anyone is paying attention. He also includes positives with his double negatives in an attempt to show the proper usages.
"Logically", he explains, "the use of two negatives in a sentence should make the sentence positive in meaning, but that is not how people talk when they say things like 'No, you cannot have that.' As this is never taken to mean, 'You may have that.' No one says, 'Yes, you cannot have that' or 'No, you may have that.' There is no way to have two positives have this problem and have a negative meaning as in, 'Yes, you may have that.'
From the back row a student replies, "Yeah, right."