Four Monkeys

Once upon a time there was a group of scientists at an animal behavioral lab. They had four monkeys in a cage. Every day, they would lower some bananas into the cage. When the monkeys tried to grab the bananas, the scientists would spray the monkeys with ice cold water. Pretty soon, the monkeys didn't try to grab the bananas anymore. Then the scientists took one monkey out and put a new one in. When they lowered the bananas into the cage, the new monkey tried to grab them but the other monkeys stopped him. Pretty soon, the new monkey didn't try to get the bananas anymore either. Eventually, all the four original monkeys had been replaced with four new monkeys that had never been sprayed with ice water, but still the monkeys didn't try to grab the bananas because they had been trained by their predecessors.

And that's why there is still open circuit technical divers who don't want to dive rebreathers!!