Today the custom is to make sure that the entire process of baking matza takes less than 18 minutes from beginning to end. As we will see, this ruling stems from a discussion in the Gemara on our daf. Nevertheless, in the time of the Mishna, dough was usually considered hametz when it showed certain signs of leavening (see daf 48).
Mishna: Deaf dough [batzek ha-heresh ] is dough for which it is difficult to determine if it has been leavened. It is comparable to a deaf-mute, who cannot communicate. If there is dough similar to it in that water was added to both at the same time, which became leavened, the deaf dough is prohibited. Although it has not shown external signs of becoming leavened, it can be presumed that the deaf dough has also become leavened.
Rashi explains the expression batzek ha-heresh as dough that does not clearly indicate whether it has become hametz and is difficult to understand, like a deaf person who has ears, yet we cannot tell whether or not he can hear.
According to the Rambam, dough that has become hametz makes a certain noise when you drum on it. The case in the Mishna is one when no such noise is heard, so it is “mute.”
The Ramban explains that just as a deaf person has trouble hearing, similarly this dough is having trouble rising.
According to the Ge’onim the expression stems from the perception of people at that time that a deaf person could not be educated and was considered “retarded.” This dough that did not follow the normal development pattern was compared to that situation.
The Gemara’s question is: What should be done if no other dough was made at the same time so that there is nothing to compare it to?
Rabbi Abbahu quotes Resh Lakish as saying that the amount of time that it takes to become hametz is the length of time that it takes to walk a mil – the distance from Tiberias to Migdal Nunia.
The opinions on the definition of this amount of time range from 18-24 minutes. Given the severity of the prohibition of hametz the usual practice, as mentioned above, is to arrange for baking to be completed within 18 minutes.