Once upon a time there was a great psychologist named Pavlov and he had a dog. He trained his dear pooch to drool whenever he would ring a bell. At the beginning of his experiment, Pavlov would ring the bell each time he set out the dog's food. Soon enough the dog learned that when the bell rang he would get food and of course would drool profusely when the food arrived. Eventually the dog would start drooling just when he heard the bell, even before he would see the food.
We have cows much like this dog. Ok... no... not really. They don't drool and there is no experiment happening on the ranch. But over time they have learned that when the feed truck drives through the gate it means feeding time. Its cool to see our cows come running when ever the feed truck goes through the gate!