Rain drops form in clouds, from really tiny water vapour particles grouping together to make bigger groups of water vapour drops. When they get too heavy to be suspended within the cloud, they fall to Earth as a raindrop. Each raindrop has taken a different length of time to form, and that’s why they fall in small drops continually, rather than all in one sheet, because the water vapour doesn’t convert into groups of water molecules all at the same time.
It’s quite lucky when you think about it, as otherwise loads of water would fall in one place, craeting flooding in one area, and droughts in others that haven’t had any rain! It would also make Glastonbury Festival even muddier than it already is 😛 I usually go, but having seen photos of the mud there this year due to all the rain, I’m so pleased I didn’t go!