Imagine you run a small ice cream shop. You know sales spike every summer and slump in winter. Last week you sold 200 cones, so you order enough for 200 cones next week. But what if next week is the start of a heatwave? A forecast that just copies yesterday misses the fact that things change over time in predictable ways. This chapter gives you the tools to spot those patterns in data collected over time—and to make smart, forward-looking decisions, not just knee-jerk reactions.