Superman is one of Calvin's minor alter egos.
Calvin is seen about to fly and is seen saying the most famous line in the Superman franchise, "Up, up, and away!", but then when he falls, he pretends that there's kryptonite near him.
Calvin tries to fly again and saying "This is a job for...", then falls again with Hobbes standing there saying "For ?". Then Calvin replied "Someone else." However, it is unknown if this alter ego is based off of Superman (see Calvin in Superhero Form).