What's really stiff and also flexible?

No, not that. The "beard" of a mussel--it's byssal threads that it uses to attach to a rock or piling.

A mussel's byssal threads are super strong, but also able to stretch up to 70% of their length. They need to be stretchy and strong to hold on during rough storms that threaten to tear it loose from wherever it wants to be, like on a wave-pounded rock or piling that supports a dock. Let go, and the mussel will likely die for lack of a good home.

So nature has evolved the incredible byssal thread, with properties that we can't create in man-made materials. It's super strong, but also able to stretch up to 70% of it's length. Wow.

Mussels are some of my favorite animals, both in the sea and also steamed in a bucket.