OK, you guys have been holding out on me. Sinew is fun stuff! Nature's duct tape.
I've been pounding up a bunch of buffalo tendons. It's quite a bit of work, but kind of relaxing and rather therapeutic. Long-term beating the daylights out of something after a hard day at work? Sign me up. And wow, it's actually fun to work with! I'm mostly going to be backing bows with it, but I've done a couple arrow fletches just for fun. I love that I can just pop a string in my mouth, get it wet, wrap it around, and it stick to itself with no need for a knot. (I've always reinforced self-nocks and tied on fletchings with silk thread, and tying it in place is by far the most time consuming part)
I also used a strand to stick together a broken arrow, again just for fun. Then I broke it again to see what would happen. The wood alongside the sinew broke, but the sinew didn't budge. This is some stout stuff! Fun to work with too. Can't wait to get some on a bow back.