One may not add flour to haroset, a seasoned, pungent food, or to mustard, to dull the sharp taste. In both cases, the pungency of these foods might accelerate the leavening of the flour. And if one added flour to either of these, the mixture may be eaten immediately before it is leavened; and Rabbi Meir prohibits this, lest the food be leavened immediately.
The mishna continues: One may not boil the Paschal lamb in ordinary liquids or in fruit juices, as the Torah explicitly states that it must be roasted [see Shmot 12:9]. However, one may baste it while it is roasting and dip it into liquid while eating it. [According to the Rambam no liquids can be added until after it has been roasted.]
The further states: Water that has been used by a baker for cooling his hands or washing dishes should be poured out, because this water leavens the dough, as the water probably contains a small quantity of flour and dough.
With regard to the first rule, Rav Kahane teaches that the argument between Rabbi Meir and the Tanna Kamma is only in the case of mustard, but with regard to other spices everyone agrees that it must be destroyed immediately. The Maharam Halavah suggests that the reason you can eat mustard mixed with flour immediately is because mustard loses its flavor if it is not eaten right away, so we need not fear that it will be left long enough to become hametz. Most other spices are used sparingly over a period of time, so it is likely that they will become hametz.
The second rule taught in the Mishna that deals with specifics of the Passover sacrifice seems to be out of place amongst the rules and regulations about hametz, which are the main concerns of this chapter. Rabbi Shlomo Adani in his commentary to the Mishna, Melekhet Shlomo, suggests that since the Mishna opened by teaching that one cannot mix flour with certain types of liquidy foods, it continued by teaching another Passover related rule – the korban Pesah itself – where mei peirot (fruit juice) cannot be used.