The skeleton of a puffer fish.
When you think nature cannot amaze you anymore, if you take a good hard look, you’ll notice that the spiky bones aren’t actually connected - they just sit under the pufferfish’s skin like caltrops and expand to spread themselves apart and point outward when the pufferfish inflates!
So when the pufferfish ISN’T inflated, these bones actually layer over each other to form a very stiff armor consisting of multiple layers of bone spikes, which is not very flexible, but it does prevent some random eel from just wandering up and stuffing them in its mouth.