You participate in a taste test for a new protein supplement called "Boost." You are given five consecutive one ounce vials of the supplement and after consuming each vial you are asked to note your reaction. You consume the first vial and your response is: "Hmmm, quite good!" After the second, you say, "Not bad at all." After the third, you note, "It's alright." and after the fourth you wince, "No more, the after-taste is getting to me. I need water." What economic principle does this scenario illustrate? Define the principle.