Beitzah, Chapter 3, Mishnah 7. This Mishnah deals with two matters connected to preparing meat on Yom Tov: sharpening a knife and weighing meat.
Sharpening a knife on Yom Tov:
The Mishnah opens: "Ein mashchizin es hasakin b'Yom Tov" - one may not sharpen a knife on Yom Tov with a sharpening stone, because this resembles repairing a utensil, which is forbidden on Yom Tov.
But the Mishnah goes on to teach a permitted method: "Aval mashiah al gabei chavertah" - one may pass one knife over another and sharpen it against it. This act is not considered actual repair, and it is therefore permitted on Yom Tov.
Weighing meat on Yom Tov:
Continuing what was taught in the previous Mishnah, our Mishnah rules: "Lo yomar adam l'tabach shkol li b'dinar basar" - a person may not ask the butcher to weigh out for him meat worth a particular coin, since one does not fix the value of an object on Yom Tov.
How, then, should they act?
The butcher slaughters, and the pieces are divided among the people. The reckoning - how much each one owes - is worked out after Yom Tov, provided that he does not mention the exact price of the pieces on Yom Tov itself.
In summary: In this Mishnah we learned that one may not sharpen a knife on Yom Tov with a sharpening stone, since this is like repairing a utensil, but one may sharpen it against another knife; and also that one may not order meat worth a set coin, but rather the pieces are divided up and the reckoning is made after Yom Tov, without mentioning a price on Yom Tov.