Imagine you have a jar filled with thousands of tiny beads, and you want to know the average weight of a bead. You cannot weigh them all, so you scoop out a handful, weigh those, and use that to guess the jar’s average. How good is your guess? What if you took a different handful? This chapter is about turning a sample into a single number — an estimate — and understanding how much we can trust that number. We will build the tools that let you move from saying “I think the average is 3.2” to saying “I am confident the true average lies between 3.1 and 3.3.”