A practical medicine exam. The first co-ed walks in. Professor: - Spit here. How, take the microscope and tell me what that is? - It's sperm, professor. - You flunk!!! Next! Next students walk in and the same question with the same sample is asked. Those answering Spit get A, those saying Sperm get F. After the last student has gone, professor decides to look at the sample himself. He finds a chunk of sperm, runs out of the room, and cries to the last student: - It's sperm! Those who had A will have F, those with F get A and tell that first student to brush her teeth in the morning!