A watch is a small clock you wear on your wrist or carry in your pocket. Before watches existed, people told time by looking at large clocks in town squares or church towers. The invention of portable watches in the 1500s changed everything, letting people carry time with them wherever they went. For centuries, pocket watches hung from chains in people's vests. Then in the early 1900s, wristwatches became popular, and now most people wear them strapped to their wrists. Today's smartwatches can do much more than tell time: they track your steps, send messages, and run apps.

The word also means to look at something carefully and attentively. You might watch a movie, watch the sunset, or watch your little brother to make sure he stays safe. When a teacher tells you to watch a science experiment closely, she wants you to observe what happens with full attention. A security guard might watch the building overnight, staying alert for anything unusual. In this sense, someone on watch is keeping guard or staying vigilant.