Here's why:

* Thin Atmosphere: Mercury has a very thin atmosphere, which means there's very little to trap the sun's heat.

* Slow Rotation: Mercury rotates very slowly (taking 59 Earth days to complete one rotation), which means that one side faces the sun for a very long time, heating up intensely.

* No Atmosphere to Regulate Temperature: Without an atmosphere, there's nothing to distribute the heat evenly, leading to extreme temperature swings.

Therefore, Mercury experiences incredibly hot days (around 800 degrees Fahrenheit) followed by extremely cold nights (around -290 degrees Fahrenheit).