[If a person] was plowing and he struck a rock or a fence, or he shook out the plow, until that point is the beis hapras. R’ Eliezer says: One beis hapras can make another beis hapras. R’ Yehoshua says: Sometimes it does, and sometimes it does not. How so? [If] one plows half a furrow, and then plows the other half or to the sides, this makes another beis hapras. [However, if] he plows a furrow’s length, and then plows from that point on, [the beis hapras] does not make another beis hapras.