My attempt to describe special relativity (from Jay)

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The special theory of relativity reconciles Galileo’s principle of relativity—the observation that the speed at which an object appears to be moving depends on the relative velocity of the observer—with evidence obtained in the late 19th Century that the speed of light is the same whether measured while moving toward a stationary light source or away from it.

In order for both of these seemingly contradictory facts to be true, Einstein deduced, the time required for an object to move from one point to another (e.g., an airplane traveling from New York to Paris) must vary depending on the relative motion of the observer (e.g., a passenger on board the plane versus a friend on the ground). This difference is too slight to be detectable for most human activities, but it becomes pronounced at extreme speeds.

Once one accepts the idea that time itself is mutable, the mathematics of special relativity are fairly straightforward and can be demonstrated using basic trigonometry. The same cannot be said for general relativity, which marries special relativity and Newton’s law of universal gravitation (the principle that objects are attracted to one another by a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them).