MIT professor Edward Lorenz made a serendipitous discovery about weather in the early 1960s, using a digital computer. He found that a small error in the initial conditions for a weather forecast could grow exponentially into a large error within days. This came to be known as the Butterfly Effect, one of the founding principles of chaos theory. To obtain initial conditions for weather forecasts, data from hundreds of weather balloons launched daily around the world are combined with satellite data through a procedure called data assimilation. The Butterfly Effect limits the useful period of weather prediction to about two weeks in advance. Carrying out an ensemble of weather forecasts, starting from different initial conditions, allows probabilistic predictions that extend beyond this limit. The philosophical concept of determinism is introduced, as is the concept of Laplace’s Demon, motivating the notion of a Climate Demon that predicts climate perfectly.