Mikvaot, Chapter Six, Mishnah Four
If a sponge or a bucket containing three logs of water fell into a mikveh, they do not make it invalid, because they have only said: “if three logs fell in.”
This mishnah is a great example of how some later rabbis read and expound upon statements made by earlier sages. The earlier sages said that if three logs of drawn water fall into a mikveh that doesn’t have forty logs of water, they render it invalid. The rabbis of our mishnah say that there must be three actual logs of water that fall into the mikveh, not a sponge or a bucket containing three logs of water. Note that the mouth of the bucket must be thinner than the tube of a water-skin. If not, we are in the situation in mishnah two of this chapter and we consider the two sources to be joined.