Researchers at the University of California at Los Angeles studying baker's yeast cells discovered that the cell walls vibrate 1,000 times per second. These motions are too slight and fast to be caught on video, but when converted into sound, they create what the scientists describe as a high-pitched scream. (It's about the same frequency as two octaves above middle C on a piano, but not loud enough to hear with the naked ear.) "I think if you listened to it for too long, you would go mad," says biological physicist Andrew Pelling, now at University College London. Pelling and Jim Gimzewski, his adviser at UCLA, theorize that molecular motors that transport proteins around the cell cause the walls to vibrate.