Birds and Bears and Bed Head:<br>
Feathers and fur and the hair on your head
(that sticks up when you get up out of bed)
and horns (but not antlers) and nails on your hand
and hooves that go galloping over the land

These all may be different but in ways are the same
although bears don't get bed head and rarely are tame.
The secret is keratin, a protein that's fibrous -
another example of polymers inside us.

It fills every hair, every feather and fur.
Just think! It's because of this cool polymer
goats can butt horns and puppies are cuddly
and horses can trot through the streets that are muddly.

Your hair will be straight or all curly in rings
Depending on crosslinks of sulfurs in cysteines.
If one side of a hair has more keratin crosslinks,
that side will be shorter, and curls (or just kinks).

Breaking these crosslinks with heat or a perm
allows us to change how we look for a term.
The crosslinks re-form in the way that we want
For a while, at least, and that's quite a stunt!

And when the keratin is really quite thick
that's how it makes hooves that go click clack click.
Keratin fibrils are even in skin
Protecting us, helping to keep all our guts in.

But you already knew, before this silly old rhyme
that polymers surround us all of the time!
grumpy bear ruffly bird
goat puppy
A cysteine is an amino acid that has sulfur in its side group: -S-H. A protein that has cysteines in it can form crosslinks called disulfide bridges, because each bridge is made up of two sulfur atoms bonded together: -S-S-. (The H atoms go away when a disulfide bridge is formed.)
chicken wild goat
camel face

A Coiled Coil

Keratin has a really cool fiber-like structure that's perfect for making things like hair. The chains are twisted into a right-handed helix, and two chains are twisted to the left around each other. This helps to hold them together.
This type of twist is called a coiled coil. Really, that's the scientific term for it! Two of these coiled coils get together, and then more of those get together, until there's a nice big bundle of little fiber-like structures that are nice and strong, like a cable. Your hair is filled with these, as is fur and feathers too.
An interesting side note is that this kind of twisting works for long hair, too. Divide long hair into two sections, and twist each section to the right. If you then twist the sections around each other to the left, it won't fall out right away (but it will fall out right away if you try twisting them to the right).

3D image of keratin coiled coil