A biology major was taking a course on cell biology. The task of the day was examining epithelial cheek cells under a microscope. The students had to scrape the inside of their mouths with a toothpick and make a slide from it, then record the different types of cells that were found. One girl in the class was having trouble identifying some cells. She called the professor over for help.

After a moment of peering into her scope, he looked up and said in a loud voice, "Those are sperm cells."