The science we learn in grade school is a collection of certainties about the natural world ─ the earth goes around the sun, DNA carries the information of an organism, and so on. Only when you start to learn the practice of science do you realize that each of these "facts" was hard won through a succession of logical inferences based upon many observations or experiments. The process of science is less about collecting pieces of knowledge than it is about reducing the uncertainties in what we know. Our uncertainties can be greater or lesser for any given piece of knowledge depending upon where we are in that process ─ today we are quite certain of how an apple will fall from a tree, but our understanding of the turbulent fluid flow remains a work in progress after more than a century of effort.