In this lesson from Yale's Rocket Science for Everyone, astronomer Marla Geha introduces the rocket equation, the fundamental relationship that governs how fast a rocket can ultimately travel given its fuel and structure. The lecture explains why rockets require such an overwhelming fraction of their mass to be fuel and why household chemical reactions cannot substitute for purpose built rocket propellant. Building on the previous lesson's introduction to solid and liquid chemical rockets, this lecture supplies the quantitative core of the course's propulsion section, setting up the following lesson's discussion of multistage rockets as a strategy for overcoming the rocket equation's limits.