In 1927, physicist Ivan Pavlov demonstrated in practice a theory that would become famous and bear his name. It basically states that the human body can be conditioned to respond in a certain way to the external stimuli it receives, and it is possible to forge a response in the body with just one stimulus: the so-called conditioned reflex. But not everyone knows that this theory applies to the so-called placebo effect of medicines. There are people, for example, who feel a headache go away before the aspirin they have taken can take effect: they just believe that the aspirin relieves the pain and the body reacts according to this stimulus.