Makhshirin, Chapter Five, Mishnah Eight
1) [Water on] the covering of tables or on the matting of bricks does not come under the law of if water be put;
2) But if they were shaken, it does come under the law of it water be put.
Section one: A person put a covering on a table or on a pile of bricks to protect them from the rain and sun. If the covering gets wet, the water does not cause susceptibility because he didn’t want it there.
Section two: However, as in yesterdays’ mishnah, if he shakes the coverings off to remove the water, then since he wanted to move the water, it does cause susceptibility to impurity.