If you ever drive through Northern France, you'll see a lot of butchers that sell horse meat. You'll also see a lot of glue factories. The two are very definitely linked — but why is it that horses make good glue?
One word: collagen. Over at Slate, there's a great explanation about the long, and oddly fascinating, history of glue-making. But what it all boils down to is that one protein, collagen. You find it in cartilage and tendons, and lurking inside bones. If you boil enough of those body parts down with some water, you get a gelatin.
Yep: that's the stuff that makes Jell-O set and Gummy bears chewy. And it makes damn good glue, too. But it's not that horse glue is actually better than any other animal glue; it's just that historically horses were plentiful, so it made sense to use them. You should definitely read the Slate pirce though — it makes for great reading. [Slate; Image: Moyan Brenn]