"Is this really a proof?" is the exact question e-mailed to me today from an undergraduate mathematics student whom I know as a highly competent student. The one sentence question was accompanied with the following demo:

I am looking for a down-to-earth, non-authoritative answer who one may give to such a student. What would be your answer if you were faced with such a question?

Update after closure. Reading the comments you may realized that most of them answer the original title of the post: "Is this really a proof?" Of course, the answer to such a question is as clear as the daylight for MO users. And, such a question should be closed asap. But, the actual question was (is) in the body of the post, and it was (is) what your constructive answer would be to such a student if you were faced with such a question. Now, with the change of the title, the actual question is much more clearer, and I hope, worthy of MO attention.