A biology major was taking a cell biology course. The task of the day was examining epitheleal cheek cells under a microscope. They had to scrape the inside of their mouths with a toothpick and make a slide from it and record the different types of cells that were found.
One girl in the class was having some trouble identifying some cells. She called the professor over to ask him.
After a moment or two of peering in her scope, he looked up and said in a loud voice, "Those are sperm cells."