Terumos, Chapter 4, Mishnah 6. The Mishnah addresses the question of how a person calibrates his measuring vessel for the purpose of separating the tithes. For Terumah Gedolah an estimate is sufficient, but Maaser Rishon must be an exact tenth, and Terumas Maaser a tenth of that, and so on. How, then, was this done in practice?
Let us illustrate this with someone picking figs: over the course of the summer the figs become drier, less juicy and smaller, as the season advances and the water in the ground decreases. What would they do? They would fill a basket with figs and count how many figs fit into it. If the basket held eighty figs, it turns out that for every basket one must separate eight figs for Maaser Rishon.
And so at the end of the harvest: if he ended up with five baskets of figs, he calculates a tenth of them - five baskets times eight figs per basket, forty figs in all. This calculation works for any number of baskets.
But here a problem arises: since the average size of the figs changes over the course of the summer, one has to recount from time to time how many figs fit into each basket, in order to adjust the calculations. The question is how often, and on this the Mishnah comes to teach three times in the harvest season when one must reassess the capacity of the vessel.
The text of the Mishnah:
"Bishloshah perakim mesha'arin es hakalkalah" - at three times in the picking season one must assess how many figs, for example, fit into the basket:
"Bibechuros" - at the beginning of the season, with the first fruits.
"Uvasayfos" - at the end of the season, with the last fruits.
"Uv'emtza hakayitz" - in the middle part of the picking season.
At each of these times a person calculates how many figs there are in each basket, and makes his calculations accordingly.
Three levels of precision in separating the tithe:
"Hamoneh meshubach" - one who counts the number of figs and gives a tenth of the total amount, as in our example forty figs out of five baskets, and does not suffice with giving half a basket by rough estimate - this is praiseworthy.
"Vehamoded meshubach mimenu" - one who measures by volume is superior to one who counts, since he separates a precise measure of volume.
"Vehashokel meshubach mishlashtan" - one who uses a scale and separates by weight, this is the best and most precise method of all.
In summary: the Mishnah teaches that the vessel is calibrated three times in the season - at the first fruits, at the last fruits and in the middle of the summer - because the size of the fruit changes over the course of the season. We also learned three levels of precision in separating the tithe: counting, measuring and weighing, with separating by weight being the ideal, since through it one gives an exactly precise tenth as is fitting.