Before we learn things formally, we have a lot of informal, experiential knowledge. This is particularly true in science. We live in a physical, chemical, biological world and we pick up how things work by virtue of our first-hand experiences. If we didn’t, we wouldn’t be able to walk or eat or make even the most basic predictions about our environments — we “know” that if we throw something, it will eventually hit the ground. We rely on the fact that our feet won’t sink through the ground. We observe that water flows downhill. We have tons of intuitive ideas about our world. But not all of our intuitions are accurate. Sometimes, we have to learn things the “hard way.” Washing hands to clean off invisible pathogens is fairly new — how could we “know” that there was bad stuff that we couldn’t see hiding under our fingernails?