The glow deceives beautifully. What passes for steam above a breakfast cup is not visible water vapor but a cloud of tiny liquid droplets, made conspicuous as they scatter sunlight toward the eye. The distinction matters here. Water in its gaseous state remains invisible under ordinary kitchen conditions, even when a sunbeam seems to catch it rising from the rim.
Cooling deserves the credit. As water evaporates from a hot drink, the resulting vapor mixes with cooler surrounding air, which can bring the mixture to saturation and allow condensation to produce suspended droplets. No boiling is required. Evaporation occurs at the liquid surface below the boiling point, so a cup can supply moisture without the bubbling associated with a kettle. Visibility has another condition. The droplets must scatter enough light toward the observer to stand out against their surroundings, which explains why a bright window and a dark background can make the same plume seem suddenly more substantial.
Disappearance is equally deceptive. When those droplets mix into warmer or drier air, they can evaporate again, leaving the water present as invisible gas rather than removing it from the room. The cloud is temporary. Its shifting edges trace a contest among cooling, mixing, condensation and evaporation, not a boundary beyond which water ceases to exist. Sunlight supplies the spectacle. Above the cup, where the white curl thins into apparently empty air, the eye loses the water long before the kitchen does.
