One who buys from a baker may tithe from hot [bread] for cold [bread], and from cold [bread] for hot, even from many forms; these are the words of R’ Meir. R’ Yehudah for-bids, for I say: The wheat of yesterday was from one [person], and that of today was from another. R’ Shimon forbids with [respect to] terumas maaser, and permits with [respect to] challah.