Uktzin, Chapter 2, Mishnah 5. This chapter takes up those portions of a food which nobody eats and which are nonetheless given the halachic standing of food because of the role they play for it: a yad, by which one grasps the food, and a shomer, which covers and safeguards it. Such a portion is treated exactly like the food it serves: it contracts tumah with it, it passes tumah on, and it joins the food in making up the k'beitzah, the volume of food required before that food can render something else tamei. Our Mishnah records Rebbi Yehuda, who takes this principle and applies it to an onion, one layer at a time.
"Rebbi Yehuda omer, shalosh klipin babatzal" - Rebbi Yehuda distinguishes three outer peels on an onion, and he rules on each one separately according to how much protection it actually provides.
"Hapnimis, bein shleima bein kedura, mitztarefes" - the innermost of the three, whether it is fully intact or whether it has been punctured, combines with the onion. Even pierced, it still lies against the onion and shields it, so it retains the status of a shomer and is counted together with the onion toward the requisite k'beitzah for conveying tumah.
"Ha'emtza'is, shleima mitztarefes, u'kedura einah mitztarefes" - as for the middle layer, everything hinges on its state. While it remains whole and unbroken, it serves as a genuine covering for the onion, and it therefore joins with it. But once a hole has been made in it, it can no longer keep the onion safe; its standing as a shomer falls away, and it does not join in toward the shiur.
"V'hachitzona, bein kach u'vein kach tehora" - the outermost layer, the paper-thin skin, is tahor in either case. It shields nothing and shares none of the properties of the onion, and so it does not become tamei at all, nor does it help make up the k'beitzah that would allow a tamei onion to render something else tamei.
Rebbi Yehaduh's three-fold division teaches how finely this halacha is calibrated: what counts is not the name "peel" but the actual service rendered. A layer that guards the food is food; a layer that has lost its ability to guard, or never had it, stands outside the food entirely.