Why does Mentos and diet Coke react they way they do?

Because of a bizarre marketing genius in Mentos (or Coke). What a wonderful plan - sell people candy and fizzy drinks, and watch them throw them both away. Of course that's better for them than eating mentos and drinking coke, but it's still bizarre. Never tried it but I doubt it only works with diet coke. Wouldn't any carbonated beverage do? Champagne?

I remember reading somewhere that it had something to do with a similar chemical structure, and that they were both trying to do the same thing in the same space, or something like that. Not really sure, it was hearsay and a while ago.