Pauly the Raindrop (Pauly the Raindrop, 1)
Pauly the Raindrop jumps off his cloud to see what it looks like under the surface. Down there it is way too dark for him though, so he travels through the roots of a tree into it's trunk and it's branches right up into the leafs from which he floats back up to his cloud.
Illustrated with many pictures. For children from two and up.