"Hey, sexy!" Millions of women and girls around the world will have heard that phrase - or something like it - cutting into their daily lives, unwanted and uninvited, as they walk to work or get the bus to school. Some might say that it's harmless, just a joke, or perhaps even a compliment. But catcalling is none of those things. It's an explicit demonstration of power, one that is intended to frighten or intimidate the person it's addressed to. It is based in deep-rooted gender inequality, which sees women's bodies as not their own.