One afternoon, the weather suddenly became gloomy. The students in Class One, Grade Nine are having a class in the classroom. There is a lot of fog on the window glass, but there is no one next door.

Answer: The weather turns cloudy, the temperature drops, and the temperature of the glass on the window drops. When the water vapor in the classroom meets the low-temperature glass, it will release heat and liquefy to form small water droplets, which will adhere to the inner surface of the glass. In the physics laboratory next door, there is no large amount of water vapor exhaled by students, so the liquefaction phenomenon on the glass is not obvious.