During a research experiment a marine biologist placed a shark into a large holding tank and then released several small bait fish into the tank.
As you would expect, the shark quickly swam around the tank, attacked and ate the smaller fish.
The marine biologist then inserted a strong piece of clear fiberglass into the tank, creating two separate partitions. She then put the shark on one side of the fiberglass and a new set of bait fish on the other.
Again, the shark quickly attacked. This time, however, the shark slammed into the fiberglass divider and bounced off. Undeterred, the shark kept repeating this behaviour every few minutes to no avail. Meanwhile, the bait fish swam around unharmed in the second partition. Eventually, about an hour into the experiment, the shark gave up.
This experiment was repeated several dozen times over the next few weeks. Each time, the shark got less aggressive and made fewer attempts to attack the bait fish, until eventually the shark got tired of hitting the fiberglass divider and simply stopped attacking altogether.
The marine biologist then removed the fiberglass divider, but the shark didn’t attack. The shark was trained to believe a barrier existed between it and the bait fish, so the bait fish swam wherever they wished, free from harm.