This document uses the analogy of a rock sinking in water while a helium balloon floats to represent acids and bases on the pH scale. It explains that if an acid and base are tied together and their densities average out to the density of water, they will neutralize each other at the middle of the pH scale (pH 7). The more basic the base is compared to the acid, the more basic the overall mixture will be, rising on the pH scale. Conversely, the more acidic the acid is compared to the base, the more acidic the overall mixture will be, falling on the pH scale.