Who knows eighty-one?
Please cite/link your sources, if possible. At some point in the next few days, I will:
Upvote all interesting answers.
Accept the best answer.
Go on to the next number.
Niddah 30b records a gruesome experiment performed by Cleopatra to determine how long it takes an embryo to develop: She took pregnant maidservants who had been sentenced to death, and examined the contents of their wombs. According to the Gemara's second version (in the name of R' Yishmael), she determined that a female embryo is not developed until day 81 (as opposed to a male embryo, which is already developed by day 41). The Sages, however, rejected this conclusion as having been established by fools (אין מביאין ראיה מן השוטים), since the experiment was not adequately controlled.
There is at least one word that appears exactly once in chumash.
At least one word appears exactly twice in chumash.
At least one word appears exactly three times in chumash.
There is a word that appears exactly eighty times in chumash.
There is no word that appears exactly eighty-one times in chumash. 81 is the smallest number with this property.