Everything about a grocery store's layout is designed to make it seem like you're spontaneously buying something that wasn't on your list. To get to the milk and bread at the back of the store, you have to pass through many other aisles of goodies. Even demonstrations and sampling carts aren't really intended to make you buy the product you taste; they're designed to slow you down and make you see all the products that are around you as you wait for your bite.