Hey Dread, a better way to approach it is ask what part of the theory of relativity Purple doubts. Actually, you can be more specific and tell us which one you are referring to (general or special?). Besides that, ask Purple what part of relativity doesn't he/she get.
I don't think we can do the theories justice by clumping them all together than refer to them as "relativity" and then challenge someone with a single sentence.
You can start by pointing out the daily observations of time dilation happening. You explain to Purple how general relativity explain's Mercury's orbit better than Newton's Laws.
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