Niddah, Chapter 6, Mishnah 3. This Mishnah opens a long series of mishnayos built on a single pattern: cases in which it is impossible to have one thing without a second thing, while the second thing can perfectly well be present without the first. The pattern has just been taught with regard to the signs of a girl's maturity, where the lower sign (the growth of two hairs) can appear before the upper sign (the development of the breasts), but the upper sign never appears unless the lower sign has already come. Our Mishnah begins with the word "kayotzei vo" - similarly, in the same vein - and brings two further illustrations of the same kind of relationship, one from the laws of vessels and one from the human body.
An earthenware vessel with a hole
"Kol kli cheres shehu machnis, motzi" - any earthenware utensil that draws liquid inward will also discharge it. Picture a pierced clay pot: once the puncture is broad enough for water to pass inside, nothing prevents water from passing back out through that very same opening. "Veyeish shemotzi ve'eino machnis" - the opposite situation exists as well. A puncture may be so narrow that moisture oozes out of it drop by drop, while nothing from outside can find its way in through the same tiny gap. Here again, the first state carries the second along with it, though the reverse is not true.
A limb with a fingernail
The second case in the Mishnah deals with a person born with a superfluous finger. "Kol eiver sheyeish bo tziporen, yeish bo etzem" - wherever a nail has grown, we may take for granted that bone lies underneath; the nail itself testifies to the bone. "Veyeish sheyeish bo etzem ve'ein bo tziporen" - the reverse arrangement is also found: an added digit may well contain bone even though no nail ever developed upon it.
Both illustrations sharpen the very same idea. A nail tells us there is bone; a bone tells us nothing about a nail. A hole that admits liquid surely discharges it; a hole that discharges liquid may still be too narrow to admit it. This is the shape of reasoning that the following mishnayos will continue to apply, case after case.