In these experimental trials on a captive porpoise at the Fjord & Baelt Center in Denmark, a trainer on one end of a pool directs the porpoise to find a fish. A D-tag on the porpoise records the echolocation sounds the porpoise uses to locate the fish. The voice yelling "Sending!" is the porpoise's cue to go. Next, an assistant at the other end of the pool slaps the water with a stick and then throws in a fish. The porpoise's task is to swim across the pool, find and eat the fish, and return to the trainer. In some of the trials, the porpoises wear eyecups, which act as a blindfold and prevent them from using vision to find the fish, so they rely solely on echolation.