I&#x27;m going to say basic germ theory. Suddenly, you can safely go about in the world as opposed to constantly getting sick via the simple act of washing up.  Didn&#x27;t washing up before delivering babies greatly lower infant mortality? Boiling water&#x2F;treating sewage make everyone healthier? Not to mention, the benefits of washing out wounds to avoid infection.<p>As a free bonus, this stuff automatically gives you an idea of microbes which should lead to lenses, microscope, telescopes, and, ideally, another scientific enlightenment.