Each child was invited into a separate room to draw for 6 minutes then afterwards either given their reward or not depending on the condition. Then, over the next few days, the children were watched through one-way mirrors to see how much they would continue drawing of their own accord. The graph below shows the percentage of time they spent drawing by experimental condition:

This is an interesting article talking about some recent research having to do with intrinsic and extrinsic motivators.

I would have expected the "Surprise Reward" group to do better than the "No Reward" group, but according to the article (and the graph above) there was no statistically significant difference.