Placebo

A placebo is a harmless pill, medicine, or procedure prescribed more for the psychological benefit to the patient than for any physiological effect. This can also be referred to as the magic feather in fiction, an item that is said to have some supernatural power attached to it in order to boost the person's confidence in their own abilities, particularly when it is later revealed that there is no actual power in the item.

Mainstream Smurfs Media
In the comic book story "The Olympic Smurfs" and the cartoon show episode "A Little Smurf Confidence", Weakling is helped by Papa Smurf giving him a placebo called Can Do Cream which is said to give the user enhanced strength, only to find out later on that it was actually smurfberry jam (or some other flavor of jam) and that his "enhanced strength" was simply his own strength being used to its full potential.