Sea urchins like to pick up shells and rocks and wear them as "hats"; the reason isn't well-understood, but it may protect them from predators or help weigh them down so they're less likely to be washed away during storms. So some aquarists have taken to making tiny hats for their sea urchins, which the urchins voluntarily pick up and put on:
@noelle urchin getting washed away, wearing a little top-hat
“sonofa~
@noelle I love this