This is a talk, aimed at the general public, about two things: how to build a language from scratch, and how thinking about this can provide us with insights into how real languages work: when we invent a language it clarifies for us what the rule of languages are that we can choose to break or not. We can also use the idea of invented languages in linguistics to see what kinds of languages people can learn, and this can tell us some very important things abut the nature of the human capacity for language.