In this podcast, Jack and Mark discuss the scientific meaning of the term "energy". Although it is a commonly used concept in everyday language today, its clear definition in physics took many years to develop and was associated with the work of a large number of scientists. However, the contributions of James Joule stand out as an example of how science makes progress even though there are formidable obstacles in the way. We owe our understanding of the relationship between motion and heat to the amazing insights and hard work of this nineteenth century British scientist.