Here I describe a bit about Einstein's great acheivement in thought. • Strong Equivalence Principle (SEP): Applies the same constraints as the Einstein equivalence principle but allows freely falling bodies to be massive objects. • Implications of SEP: Requires the gravitational constant to be the same everywhere and every when in the universe, and forbids any extra fields beyond the standard model of gravity, electromagnetism, weak nuclear force, and strong nuclear force. • Relationship with Einstein Equivalence Principle: Similar to the Einstein equivalence principle in requiring gravity to be geometrical in nature, but with the added restriction of forbidding any extra fields. • Strong Equivalence Principle: Matter in all flat spaces in the cosmos behaves and interacts in the same ways. • Theories of Gravity: Einstein’s theory of general relativity is the only theory that satisfies the strong equivalence principle. • Applications of the Strong Equivalence Principle: It is sufficient for almost all studies of the universe’s evolution and fate, from the Big Bang to the future. • Equivalence Principle in Cosmology: The strong equivalence principle is likely applicable to most cosmological observations. • Hubble Tension and Potential Violations: The ongoing debate about the Hubble tension suggests potential violations of the strong equivalence principle. • Exploring the Equivalence Principle: The lecture will delve into examples and thought experiments to better understand the equivalence principle.