Scientists believe they have discovered why. For the first time, experimental rat studies have shown the brain process that converts unpleasant experiences into long-lasting memories. The research also reveals how blocking this mechanism prevents rats from forming painful memories. The study, published in Science magazine, showed that when glutamate - a chemical in the brain responsible for memory and learning - interacts with another protein, it creates a cascade of reactions that transforms an innocuous experience into a memory that will always cause pain. Glutamate is like a key that opens the door to the memory storehouse inside our brains. Without its interaction with this other protein, it can't do so. The researchers blocked this protein using an enzyme called gamma-secretase. They knew this would prevent the formation of painful memories, because previous work had already shown this enzyme to be necessary for creating long-term memories. Thus, they expected the rats to not form pain memories after their experiences. But what they found was surprising: Even though these rats were given pain shocks every time they entered a room where they'd previously been burned by hot water, they didn't develop any long-lasting feelings toward the heat source. They didn't run away from it or avoid places where it had been set off before. They simply didn't remember that the water was hot.