This resource outlines a classroom activity focused on the concept of the expanding universe, using a balloon model to illustrate the relative motion of galaxies. Students observe and measure the distances between marked points (representing galaxies) on an inflating balloon, which exemplifies the Cosmological Principle. They will learn about the homogeneity and isotropy of the universe, and how the space between galaxies expands rather than the galaxies moving through space. The resource includes setup instructions, measurement techniques, observation tasks, and a discussion on predictions based on the 'further is faster' rule.