Googleshng:
Because he wants to go outside.

Googleshng:
Because he's been cooped up inside all day.

Googleshng:
Because it's been raining.

Googleshng:
Because enough water has evaporated into the air for it to form droplets too heavy to remain in clouds.

Googleshng:
Because the water was heated and then cooled.

Googleshng:
Because solar radiation excites molecules it strikes, and the planet is spinning around the sun.

Googleshng:
Because enough debris from the big bang congregated together to form the planet and sun.

Googleshng:
Because that's how gravity works.

Googleshng:
Because space is curved.

Googleshng:
Hey, do you want a cookie?