Rewards may have negative impacts if you're not careful.

Let us begin with a real-life example. In order to make George develop a good reading habit, his mother set up an incentive plan for him: he would receive two dollars every time he successfully finished a book. At first, George was very enthusiastic and devoted to reading. But after a while, George gradually lost his interest and patience. Until one day, George took out four dollars and gave them to his mother: “You can have these four dollars. I don't want to read for these two days.” Since then, no matter what, George wouldn't touch those books. His mother’s incentive plan ultimately failed