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Глупенький/ая. Дурачок/дурочка...

Interesting. I didn't know this. I don't see the way how to say it in russian without an insult. Technically "вы глупы"/"вы дурак" is a polite form of "ты глуп"/"ты дурак" but you can get a hook or...

Don't agree with that. "Заниматься" can be used as "learn" or more exact "be in process of learnig (something)".