Why are days on Mercury hot and nights are cold?

Mercury has nearly no atmosphere. Without an atmosphere, Mercury cannot reflect sunlight or trap heat. This is the reason why days on Mercury are very hot and nights are very cold.

In the daytime, the solar energy directly strikes the planet's surface because there is no atmosphere to reflect heat, making a sizzling temperature of 427 Celsius (800 Fahrenheit).

At night, without an atmosphere to trap heat, the solar energy escapes quickly into space, resulting in a freezing temperature of -173 Celsius (-290 Fahrenheit).