After a trip to the Amazon jungle, President Teddy Roosevelt famously reported seeing a pack of piranhas devour a cow in a few minutes. It must have been a very large school of fish—-or a very small cow. According to Ray Owczarzak, assistant curator of fishes at the National Aquarium in Baltimore, it would probably take 300 to 500 piranhas five minutes to strip the flesh off a 180-pound human. But would this attack even happen?