Toharot, Chapter Three, Mishnah Four
This mishnah deals with food that originally was large enough to be impure (or to have some other rule apply to it) and then became smaller because it was left out in the sun.
1) If an egg’s bulk of food was left in the sun and it was lessened, and so also in the case of an olive’s bulk of corpse, an olive’s bulk of carrion, a lentil’s bulk of a sheretz, an olive’s bulk of piggul, an olive’s bulk of notar, or an olive’s bulk of forbidden fat they become clean;
2) Nor is one liable on account of these for transgressing the law of piggul, notar or forbidden fat.
3) If they were then left out in the rain and they swelled, they become unclean and guilt is incurred on account of them for transgressing the law of piggul, notar or forbidden fat.
Section one: In all of these cases something was the requisite size to convey impurity or to be impure. It was then left in the sun and it dried out and thereby lost the requisite size. Since the object is no longer large enough, it becomes clean. I will explain what each of these items is:
Food: needs to be the size of an egg to become impure.
Pieces of corpse, carrion (an animal that was not properly slaughtered) and a sheretz (a creepy crawly thing): needs to the size of an olive to transmit impurity.
Piggul: is sacrificial meat slaughtered by a priest who had the intent to eat the meat in the wrong place or at a time when it couldn’t be eaten.
Notar: sacrificial meat left over beyond the time when it should have been eaten.
Forbidden fat: helev, the forbidden fat of a permitted animal.
Section two: Just as the piggul, notar and forbidden fat do not convey impurity if their bulk becomes less than an olive, so too one who eats them is not liable for karet, which is the usual punishment for eating one of these substances.
Section three: If any of these substances is left out in the rain, it returns to its original impurity and if one eats one of the latter substances (piggul, notar or forbidden fat) he is liable.