The Institute for Marine Mammal Studies had the dolphins clean their own water tanks. By turning in a piece of debris, each dolphin would be rewarded with a fish.
The caretakers thought they were pretty clever, not having to do the dirty work themselves. But after a few days they noticed that they were distributing a lot of fish and the pieces of litter were getting smaller and smaller.
A film camera was set up to catch the cheaters. Upon review, the staff notified the caregivers that they had been duped. When no one was looking, the dolphins were tearing any debris they found into little pieces with their teeth and placing the scraps under a rock until it was time to claim their rewards.
Never trust an animal that always sleeps with one eye open.