The smokers in Harry's group felt fewer cravings than expected, and the fake surgery patients felt less knee pain than expected. In both cases, may be just the idea of getting treatment (a supply of nicotine or an operation) had something to do with the results. Or perhaps something else was happening. For example, maybe the sugar in the chewing gum helped Harry and the others in his groups feel better. Scientists agree that further research is needed. So far, no one has really proven that the placebo effect exists.