Years ago, there was a joke that if a horse got injured on the track or was too slow to race, he or she would be sent to the glue factory. Where did that even come from? Well, horses have a lot of collagen, an essential protein in connective tissues as well as hides and bones. It’s also the key ingredient in most animal glues, because it can be made into a gelatin that’s sticky when wet but hardens when it dries.