Never trust a
tool-using mammal.
After years of feeding the otters pre-shucked shellfish, the staff of the Vancouver Aquarium thought it would make an interesting educational display to have otters opening their food as they do in the wild - changed the otters' diet to in-the-shell mollusks and bivalves and provided rocks with which the furry li'l guys could bash the invertebrates open.
The attendants observed the otters briefly to determine that yes, their instincts were kicking in and they were recalling the subtleties of getting at the rich chewy centres. Confident that the public would eat the spectacle up, they closed up for the night and went home.
The next morning the aquarium staff were treated to a bit of a rude surprise. Would you give skeleton keys to dangerous maximum-security prisoners in jail? If not, then you might think twice before giving heavy stones to clever animals separated from freedom only by a thin wall of glass.
The otters were located and retrieved from the surrounding Stanley Park. The public was treated to the sight of a closed exhibit until the glass was replaced, at which time it featured somewhat grumpy otters consuming pre-shelled seafood.