A factory’s chimney pours smoke onto a neighbour’s laundry-drying yard. A nightclub’s music shakes the walls of nearby flats. A farmer’s pigs create a smell that spoils a neighbour’s Sunday lunch. The law calls these clashes nuisances — but at their heart they are about externalities, invisible threads of harm that tie one person’s choices to another’s well-being. This chapter shows you how judges can untie those threads with two simple tools — an order to stop, or a requirement to pay — and why the choice between them shapes the world around us.