ELI5 Birthmarks - what causes our skin to have them?

Why do we get birthmarks? How about Moles? What type of birthmark did Mikhail Gorbachev have? What were some superstitions around birthmarks from the past?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: chabafred, bradjudy, reesuh, platinumgamer55 & bertnod

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