The early 20th century marked a number of transformational experimental and theoretical discoveries in physics. Among them is one that is often neglected in the introductory physics curriculum, which revolutionized our understanding of the molecular world. Evidence for the thermal motions of atoms was first observed by Perrin in 1909, which had been based on the predictions outlined by Einstein in his doctoral dissertation in 1905. Perrin's experiment was the first to establish a value for the fundamental constant Avogadro's number, which is the number of atoms in a mole. The theoretical underpinnings of Einstein's work are an elegant example of the connection between mechanics and thermodynamics and its application to atomic motion. The experiments of Perrin are easily accessible to undergraduate labs. The conceptual simplicity of the topic and its foundational application to chemical diffusion and thermodynamics advocate strongly for its incorporation into the introductory physics sequence and particularly for students in the health sciences.