A lifeform once resided on a human colony planet that resembled rain. Rose Tyler described it as water that was "alive".
Biology
The rain appeared to be indistinguishable from water, though when it was being boiled to death, it was able to scream. When it was captured in a saucepan, the rain was able to furiously move about on its own. (PROSE: Pitter-Patter)
Abilities
The rain could easily cut through human flesh and if it concentrated enough, it could break through metal shelters within days. It could fall in an almost melodic fashion to draw victims into the open.
The rain was able to focus its fall like a funnel to concentrate its downpour and could stop falling from the sky at will. (PROSE: Pitter-Patter)
History
When the human colonists arrived on the planet, they drank the rain, thinking it was ordinary water. In revenge for killing their kind, the rain attacked, slicing through twenty colonists and causing the survivors to run for the mines.
The rain attacks the Ninth Doctor. (PROSE: Pitter-Patter)
Downpours lasted for days at a time, not ending until at least one or two people were killed. The rain fell in what appeared to be funnels on the colonists' shelters. Six weeks later, when there were only a few survivors left, the Ninth Doctor and Rose arrived on the planet, and were attacked by the rain. They were saved when Jack and Susie let them into their bunker.
After the rain breached through the Jacksons' shelter, the Doctor went outside to negotiate with the rain. The rain stopped to allow Jack, Susie and their son, Andy, to leave for their spaceship and go back home. (PROSE: Pitter-Patter)