How so? [If] an olive-pressing basket was placed on top of a pit or a cistern that is inside a house, [the contents of the pit or cistern] remain tahor. [However,] if an olive-pressing basket was placed on top of either a well that is level [to the floor of the house] or a broken basket, [the contents of the well or the basket] become tamei. [On the other hand,] if a flat board or a kneading board without sides [was placed on top of either a well that is level to the floor of the house or atop a broken basket, the contents] remain tahor. This is because utensils combine with the walls of a structure to curtail [tumah] only when there are walls. How high must the wall be [to block tumah]? One handbreadth. [However, if] half of a handbreadth is from [a wall] and the other half of a handbreadth is from [the olive-pressing basket], it is not considered to be a wall. [It is only a wall] when one of them is a handbreadth by itself.