I heard about this experiment. It said that the kids who persevered and came through without giving in in order to get the reward, were more likely to be successful in the future. Don't know if that's true though.

This reminds me of a present my Grandfather gave me for Christmas one year. He gave me and my sister a 3D puzzle that had a 20 dollar bill inside it. When you solved the puzzle the bill would be freed. He told us that the next time he came to visit he would check the puzzles and if the 20 was still there he would double it. I was excited but he hardly ever visited us because he lived across the country. I couldn't take it anymore so my mom told me to solve the puzzle and then when we saw my grandfather again she would give me another twenty to put in the puzzle. It might not have been the best lesson for me.