Frequently, people don't understand why expansion means that light gets redshifted. Here I link the two ideas. What causes the redshift of light, and how do represent it mathematically? What exactly gets stretched? Where does the energy go when it gets redshifted? It is critical to understand these principles because without this understanding the entire baseline of cosmology is hard to get. Everything is a measurable, and redshift is one of those things. For from a mystery or a trick of the math, it’s a real occurrence. As part of this, we study the expansion in great detail, outlining the Robertson-Walker Metric, a solution to the Einstein Field Equations of General Relativity. This is part of my complete intro Astronomy class that I taught at Willam Paterson University and CUNY Hunter. Supplement the videos with OpenStax Astronomy 29: The Big Bang Redshift: Hubble's Law: Cosmic Scale Factor: FLRW Metric: 0:00 Introduction 1:38 The photon IS the wave... 3:01 Photons get emitted by shaken charges... 4:02 Measuring the Changes of Photon Wavelengths 4:45 How Does Redshift Measure Expansion? 6:21 No Curvature to the Universe 6:42 Photons Travel on Null Metries 7:25 Super-Simple, Photon-Only FWR Metric 7:53 Getting Ready for the Magic Spell... 8:31 Big Summation Time! 10:15 Comparing Two Wavecrests 10:40 The Difference Between Start and Finish 12:15 The Universe Expands Only Big Scales 13:16 But That's Redshift! 14:40 Redshift Measures the Change in Size.