They took 1 barracuda and 1 mullet and put them in the same fish tank - which was divided with a clear glass window, so each fish could not swim to the other side of the fish tank where the other fish was.
Lets call the barracuda Barry and the mullet Molly.
Barracudas eats mullets - and they REALLY love mullets...
As soon as the Barry saw Molly he cruised thru the water to eat him - but was stopped by the glass window that separated the tank in 2 halves...
Barry tried again...and again...and again - each time he was stopped by the glass window/plate.
Over the next few weeks Barry got at very sore nose and stopped hunting Molly.
The scientists then removed the glass window that separated the tank in 2 sides... and Molly swam happily around in all the tank.
Even when Molly was just a few inches away from Barry - then Barry did not try to hunt him at all.
Why is that?
Well, Barry had learned that hunting mullets equals pain - so Barry would rather starve, even though nice fresh food was just inches away from his razor sharp teeth.
What can you learn from this, really - it is just a fish, right?
Well, we all have our own fish tank we live in - this "fish tank" is determined by our beliefs, which again are largely made up of what society, teachers, parents, friends, lovers, family, bosses, etc have told us to believe, as well as our own "experiences" (like Barry's).
So, if Barry could completely change his natural behavior from hunting mullets to not hunting mullets - then it is quite likely that the same thing happens for all of us, to some degree.