Complete the camper's spooky story about a midnight visitor.

I was frozen in my sleeping bag when I heard something ___ heavily right outside my tent.

The correct answer is breathing.

After verbs of perception (like hear, see, feel), we use the -ing form to describe an action that was already in progress when we noticed it.

We never use the "to-infinitive" (to breathe) or a conjugated verb (breathes, was breathing) directly after the object in this structure.