Analyzing Jokes

The teacher of a high school biology class had his students analyzing bacteria under the microscope. To show them the types of bacteria found in the human mouth, he had them take scrapings from between their own teeth and analyze these scrapings under the microscope.
One girl was stumped in identifying the bacteria she scraped from between her teeth.
The teacher took a look, stared disgustedly at the puzzled girl and said, "Those are sperm cells."