In 1961, the Nobel prize winning theoretical physicist Eugene Wigner proposed what is now known as the 'Wigner's friend' thought experiment as an extension of the notorious Schroedinger's cat experiment. In the latter, a cat is trapped in a box with poison that will be released if a radioactive atom decays. Governed by quantum mechanical laws, the radioactive atom is in a superposition between decaying and not decaying, which also means that the cat is in a superposition between life and death. What does the cat experience when it is in the superposition? Wigner sharpened the question by pushing quantum theory to its conceptual limits. He investigated what happens when an observer also has quantum properties.