Humidifier's output

VAPOR

Think of what you see rising from a hot cup of tea or a boiling pot—it’s the same invisible substance a humidifier adds to dry air.

A humidifier adds moisture to the air, and its output is the visible mist or gas it releases. The answer "VAPOR" fits because it describes water in its gaseous form, which is exactly what a humidifier emits. There’s no wordplay here—it’s a straightforward definition where the clue directly points to the result of the device’s function.