How does soap actually get things clean?

Water alone can't wash off grease, because oil and water don't mix. Soap solves that with a molecule that belongs to both worlds. One end, the head, is attracted to water; the other end, a long tail, is attracted to fat. That makes soap a surfactant.

When you lather up, the tails dive into the greasy dirt while the heads stay pointing out into the water. The soap molecules gather around each speck of oil into tiny spheres called micelles, with the oil tucked away in the middle and a water-loving surface on the outside. Wrapped like that, the oil can be carried off by running water. Soap is also tough on germs: it disrupts the fatty membranes of microorganisms and scrambles their proteins. That's why the US Centers for Disease Control and Prevention recommends scrubbing with soap for at least 20 seconds, whatever the water temperature.

Handwashing now seems obvious, but it wasn't. In 1847, Ignaz Semmelweis noticed that in his Vienna hospital, women treated by doctors died far more often than those cared for by midwives. He suspected the doctors were carrying 'cadaverous particles' on their hands from the autopsy room. When he made them wash their hands in a chlorinated lime solution, maternal deaths fell from 18% to under 2%. Many doctors were offended and mocked him.