Nazir 43A

בבית נמי, כיון דאעיל ידיה ⁠— איסתאב, כי עייל כולי, האי טמא הוא!

The Talmud asks: With regard to one who enters into a house too, why should he be liable twice? Since one typically enters a place with his hands before his body, once he inserts his hand he immediately becomes ritually impure. This means that when all the rest of him enters, this person is already impure.

אלא אמר רבי אלעזר: צירף ידו, משום טומאה ⁠— איכא, משום ביאה ⁠— ליכא. וצירף גופו ⁠— טומאה וביאה בהדי הדדי קאתו. הא אי אפשר דלא עייל חוטמו ברישא, ונחית ליה טומאה!

Rather, R' Elazar said: If he inserted his hand into the house first, there is liability due to contracting ritual impurity; however, there is no liability due to entering the enclosure. But if he joined his body and his hands, i.e., all of him entered at once, contraction of impurity and entering the enclosure occur simultaneously, and in that case he is liable twice. The Talmud objects: It is impossible that his nose would not enter first, and once it does, impurity would descend to it and thereby to this person immediately, before the rest of his body entered the house.

אלא אמר רבא: הכניס ידו, משום טומאה ⁠— איכא, משום ביאה ⁠— ליכא. הכניס גופו ⁠— טומאה וביאה בהדי הדדי קא אתיין. והא אי אפשר דלא עייל אצבעתא דכרעיה ברישא ונחת להו טומאה!

Rather, Rava said: If he entered with only his hand there is liability due to contracting ritual impurity, but there is no liability due to entering an enclosure with a corpse, as he cannot be considered inside the house. If he entered with his body by standing upright so that his head would not enter first, contraction of impurity and entering the enclosure occur simultaneously. The Talmud asks: But even so, it is impossible that his toes would not enter first, and once they do, impurity would thereby descend to them, causing him to become impure before his entire body enters the house.