Yadayim, Chapter Two, Mishnah Two
1) If he poured the first water over his hands [while standing] in one place, and the second water over his hands [while standing] in another place, and a loaf of terumah fell on the first water, the loaf becomes unclean.
a) But if it fell on the second water it remains clean.
2) If he poured the first and the second water [while standing] in one place, and a loaf of terumah fell onto the water, the loaf becomes unclean.
3) If he poured the first water over his hands and a splinter or a piece of gravel is found on his hands, [his hands] remain unclean, because the latter water only makes the first water on the hands clean.
4) Rabban Shimon ben Gamaliel says: if any creature from the water [was on the hands while they are being cleaned] they are clean.
Section one: The first water that pours off his hands is impure. Therefore, if a pure loaf of terumah falls onto this water, it is impure. But the second water is pure and purifies the water on his hands. Therefore if the loaf falls onto this water, it is pure.
Section two: In this case, both the first and second water fall onto the same place on the ground. If a loaf falls into this water, it is defiled because the second water cleanses only the water that is on his hands. It does not cleanse the water that is on the ground.
Section three: In this case there is something on his hands when he washes them. The second washing purifies only the water that is on his hands, not the water that is on something else, such as a splinter or piece of gravel. This water remains impure and it would go back and defile his hands.
Section four: Rabban Shimon ben Gamaliel seems to think that creatures that are from the water, like a little worm, count as if they were water. So if one finds a worm on his hand after washing, the water that’s on the worm doesn’t go back and defile his hands. It might be a bit yucky though.