At the molecular scale, everything is effectively made of springs (molecular and intermolecular bonds). Proteins are a big complicated tangle of springs, and th

At the molecular scale, everything is effectively made of springs (molecular and intermolecular bonds). Proteins are a big complicated tangle of springs, and that means they have multiple possible configurations they can spring to if you let them relax. If you bump another protein of the right shape against them, you can nudge them into a different spring configuration. Think of "slap bracelets", those curved metal strips that spring around your wrist. If you push two straight ones together, nothing happens. But if you push a curled one against a straight one it will make the straight one curl up too. The curled slap bracelet is acting like a prion.