 Dozens and dozens, the Germans first and then the British later, dozens of materials that might hold hydrogen. The best they could come up with was cattle intestines, particularly the cecum, a part of the intestine. And for you and me, we would think about sausage casings. Just think about those sausages you had for breakfast the other day. That's what it is, which is astonishingly fragile. Where do you get them from? Well, this...