ELI5 Chocolate - milk, dark & white, what's the difference?

Why do Oreos get soggy in regular milk but not in chocolate milk? What is the difference between dark chocolate, milk chocolate and white chocolate? What is the % number I see on chocolate packaging and labels?

 

... 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: eliteanaconda57, thielio, rtap15, irythros, krystar78, xerloq, barefootess, clevalandfansadface & thejayhook

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