"Fill with air" is a common phrase used to describe the act of inflating something, such as a balloon, tire, or air mattress. There are several synonyms that can be used in place of this phrase, including "inflate," "blow up," "pump up," and "swell." Each of these synonyms communicates the same idea of adding air to a container or object in order to make it larger and more buoyant. Additionally, these synonyms can be used in a variety of contexts, from describing the process of filling up a beach ball, to explaining the intricacies of inflating a hot air balloon.