The phrase "catches on fire" is often used to describe the process of something igniting and bursting into flames. However, there are many synonyms that can be used to convey the same meaning. For example, you could say that something is ablaze, alight, or ignited. Other options include burning, flaming, and smoldering. Each of these words can help to create a vivid image of a fire starting and spreading. When choosing which synonym to use, consider the context of the situation and the level of intensity you want to convey.