A lighthouse is not saying “danger”. It is saying its name. Each light along a coast flashes to its own rhythm, two quick flashes every ten seconds, say, or one long one every fifteen, and the pattern is printed in a book on the chart table, so a sailor at night can count in the dark and know exactly which headland that is. By day the towers do the same thing with paint: stripes, bands, a chequerboard, so that no two neighbours look alike. All that glass and clockwork, and the whole message is a single word said over and over until morning. It turns out to be enough. Once you know who is talking, you can work out where you are.