Can you explain to me the smell of the rain?

In the beginning of spring when the flowers are popping up and the pavement is full of sun, and the air is warm; the barely-there breeze brings the smell to my nose, but the cool rain drops fall straight onto the ground. The sound is more beautiful than music, the droplets cling to the tiny twigs and branches on the trees, eventually loosing their grip and falling down into the puddles that cover the sidewalks and the ground. We know why it makes sound, why it feels cool and wet to the touch, but why does it smell like… rain?