Rashi on Genesis 1:8/רש"י על בראשית א׳:ח׳

1ויקרא אלהים לרקיע שמים AND GOD CALLED THE EXPANSE HEAVEN — The word “שמים”, Heaven, may be regarded as made up of שא מים “Carry water”, or שם מים “There is water”, or אש ומים “Fire and water”. He mingled fire with water and of them He made the heavens (Chagigah 12a).