The sky is blue because it's usually about 30% to 90% water vapor, which scatters small wavelengths of light. Clouds are areas of the sky where humidity, pressure, and temperature combine to allow the water vapor to condense enough that all visible light is scattered.

Just FYI.

I know this ELI5 answer because my son asked me why the sky is blue, way back in 1986, when he was 5.

He must have explained it to his son, because my grandson didn't ask me.