Makhshirin, Chapter Five, Mishnah Five
1) If one put his hand or his foot or a reed into a cistern in order to check whether it had any water, it does not come under the law of if water be put;
a) But if to ascertain how much water it had, it comes under the law of if water be put.
2) If one threw a stone into a cistern to check whether it had any water, [the water] that was splashed up does not come under the law of if water be put, and also [the water] that is on the stone is clean.
Section one: In the first case he just wants to see if there is any water in the cistern, but he doesn’t need to see how deep the cistern is. Since he doesn’t need to see the mark that it makes on his foot, hand or reed (see Rabbi Akiva’s opinion in yesterday’s mishnah), the water is not “to his will” and it doesn’t cause susceptibility.
However, if he does need to check how much water is in the cistern, then the water that comes up on his hand, foot or reed does cause susceptibility, as we learned in yesterday’s mishnah.
Section two: In this case, he didn’t want to bring up any water or cause any splash. He just wanted to hear the sound of the rock hitting the water. Therefore, the water that does come up and the water that is on the rock (if half of it is sticking out of the water) does not cause susceptibility.