Taharos, Chapter 4, Mishnah 9. Our chapter works through cases of safeik, doubt, in tumah and taharah, some of which the chachamim ruled tamei and some of which they ruled tahor. The mishnah before us takes up liquids, and it splits neatly in two: the first half deals with a doubt about liquids becoming tamei, and the second half with a doubt about liquids conveying tumah to something else. The two halves are judged in opposite directions.
A doubt about liquids becoming tamei
"Safeik mashkin letamei, tamei." When the doubt concerns liquids contracting tumah, the doubt is resolved stringently and the liquids are tamei.
"Keitzad?" The mishnah supplies an illustration. A man who has himself contracted tumah "pashat et raglo", stretched out his foot, toward a spot where liquids that were tahor were standing: picture open bowls of clean water or juice. Afterward it cannot be determined whether the foot actually met the liquid, "safeik naga", or never reached it, "safeik lo naga". The verdict is "sfeiko tamei": we assume that contact occurred and the liquids are now tamei. Why so strictly? Because the ability of liquids to take on tumah is min haTorah, and when a Torah level law hangs in the balance we do not settle a doubt in the direction of leniency.
The mishnah adds a second scenario that yields the identical ruling. Here the man is carrying in his hand a "kikar temeiah", a loaf of bread that has already become tamei, and he flings it, "uzrakah", into the area where the tahor liquids are standing. The tumah is now traveling in the bread rather than in the man's limb. Whether the loaf reached the liquids nobody can say, "safeik naga" or "safeik lo naga", and once more the liquids are declared tamei.
The commentators dispute what this second case adds. One approach says that the new variable is the motion: the loaf was in flight, and that is what distinguishes it from the case of the outstretched leg. The other approach says that the motion is beside the point, that we are speaking of the loaf after it has come to rest, and the case is essentially identical to the first, except that the tamei object is a piece of bread rather than a person's foot.
A doubt about liquids conveying tumah
"Uletamei, tahor." When the doubt concerns liquids imparting tumah to something else, the doubt is resolved leniently and we rule tahor. Why the difference? Because liquids receiving tumah is de'oraisa, while the power of liquids to convey tumah onward is only derabbanan, and in a rabbinic matter we are prepared to be lenient in a case of doubt.
"Keitzad?" What is the case? "Haysah makel beyado, uveroshah mashkin temeiin, uzrakah levein kikaros tehoros." He was holding a stick, and on the end of the stick were tamei liquids, and he threw it in among pure loaves of bread. "Safeik naga, safeik lo naga", it is unclear whether the liquids on the stick touched the loaves or not. "Sfeiko tahor", the doubt is resolved leniently, and the loaves remain pure.
So the same word, safeik, yields two opposite verdicts in a single mishnah, and the dividing line is the level of the law involved: strict where the Torah itself is at stake in the liquids becoming tamei, lenient where only a rabbinic enactment is at stake in the liquids passing tumah along. And the pure loaves stay pure and fit to eat.