Sinew is the connective tissue that attaches an animal's muscles to its bones. It is very strong and fine. It is used for thread, to make bowstrings, and strengthen the backs of bows. It is good for making arrows, etc. It is a bit tricky to knot.
Hair varies greatly in strength, length, texture and usability.
Curly hairs spin into the best yarn, while very straight hair wants to slip apart. Single human hairs are recorded as quail snares. You've just collected a bunch of willow stems for basket making and you want to use one of the sticks to tie the bundle together. Wrap the thin end around, passing next to the protruding end. The next time around, bend the protruding end over and wrap into the bundle.