משנה על כלים 1:4

If rain came down upon a person, even if he was unclean with a father of impurity, it does not come under the law of ‘if water be put’; But if he shook it off, it does come under the law of ‘if water be put’. If one stood under a rain-pipe to cool himself or to wash himself, [the water falling on him] is unclean if he is unclean; If he is clean, it [only] comes under the law of if water be put.

If a pot full of water was placed in a mikveh, and a person who was unclean by a father of impurity put his hand into the pot, it becomes unclean. But if [he was unclean] by a "touch of defilement," the pot remains clean. But any of the other liquids [contained in the pot] becomes unclean, for water cannot purify the other liquids.