In North America, meeting up with a porcupine can have painful consequences. It’s barbed quills are known to easily penetrate skin, and are very difficult to pull out. Researchers at Harvard Medical School have found just how easily they penetrate; a quill needs just half the force of a standard hypodermic needle. Why? Looking at the shape of the quill, the barbs fold back, allowing the arrow shape to dig into skin, but catching on to the same skin when pulled backward.