The placebo effect is famous; a non-medicinal substance or procedure can actually relieve pain, fever, or nausea even if it's "just a sugar pill." But why does this work? In this fascinating video from YouTube's Ri Channel, Professor Nicholas Humphrey tackles the question: "If we can effectively cure ourselves by taking placebo medicine, why don't we just heal ourselves straight away?"

Starting with Stonehenge (which might be a giant stone placebo), Humphrey takes us on a logical journey. Enjoy: