Why does blowing on hot food cool it down?
The short answer
Blowing moves the warm air above the food, making it easier for heat to leave the food and enter the surrounding air. With watery foods such as soup, blowing also helps water evaporate. That evaporation uses energy from the food, so it can cool down faster.
In 30 seconds
Cooling does not mean that heat disappears. Thermal energy moves from the food to its surroundings. Blowing replaces the warm, humid air at the surface with moving air. Water at the surface can also become vapor, and that change needs energy, which comes partly from the food.
What does it mean for food to cool?
When a warmer object meets something cooler, thermal energy moves from the warmer object to the cooler one. In hot soup, energy moves from the soup into the bowl and the air, so the soup’s temperature falls.
What happens when you blow?
A layer of warmed air sits just above the surface of soup or another hot food. When the air is mostly still, this layer can slow the movement of heat away from the food.
Blowing moves that air and brings other air into contact with the surface. Heat transfer caused by moving air is called convection.
Evaporation helps too
Some water at the food’s surface changes from a liquid into water vapor. Evaporation requires energy, and some of that energy is taken from the food. The food that remains therefore becomes cooler.
Blowing moves away air that already contains water vapor, so more water can evaporate from the surface. The effect depends on the food, its exposed surface area, and how humid the surrounding air is.
A common misunderstanding
It is not always correct to say that food cools because your breath is cold. Exhaled air can be warmer than the room. Blowing can still cool hot food because it moves the air at the surface and strengthens convection and evaporation.
In fact...
For the same reason, stirring soup or spreading food onto a shallow plate can make it cool faster. A larger exposed surface gives heat and water more ways to move into the air.