On a cloudy day, the sun doesn't actually change in any way. It continues to shine and radiate energy as it always does.

Here's what happens:

* Clouds block sunlight: Clouds are made of water droplets or ice crystals. These particles scatter and absorb sunlight, preventing it from reaching the Earth's surface directly. This is why the sky appears grey or white during cloudy days.

* Some sunlight still reaches the Earth: While some sunlight is blocked, not all of it is. Some sunlight still manages to penetrate the clouds, which is why we still experience daylight even on cloudy days.

* Temperature differences: Due to the reduced sunlight reaching the Earth's surface, cloudy days tend to be cooler than sunny days.

So, the sun doesn't change on a cloudy day, but the clouds affect how much sunlight reaches the Earth's surface.