What's a nice way to say you're DUMB?

Anyone? How can I, Gloomy Doom, politely and without being condescending tell a person they are the dumbest person alive? How can I explain myself, without breaking it down to the very last detail, on why I believe they are dumb? What? What is that you say? Hmm, sounds interesting. Yes, I know I can handle that. I, Gloomy Doom, with utter conviction, will utter the words with warmth: "You're Dumb. Try to Change. If not, Gloomy Doom will become your fate." And gloominess is what life is all about. Embrace the doom of the future.