Ohalot, Chapter Twelve, Mishnah Four
1) [With regard to] the shoe of a cradle, for which a hole had been made [in the ceiling to bring it] into the house [below],
2) If [the hole] is one handbreadth square, everything becomes unclean;
3) But if it was not [one handbreadth square] its [uncleanness] is reckoned as one reckons with [cases of contact with] a corpse.
Section one: To hold a baby’s cradle in place they would make a sort of shoe for the cradle and then attach the shoe to the ground. In this case the cradle is on the upper floor of a house and the bottom of the shoe can be seen in the bottom floor. In other words, they made a hole in the ceiling (of the bottom floor) to attach the cradle to the floor (of the upper story). There is a source of impurity below in the house.
Section two: If the hole is at least one handbreadth square then the contents of the upper floor are impure because the impurity can enter through a hole of that size.
Section three: However, if the hole is less than one handbreadth then the shoe that hangs over the corpse impurity has the same impurity level as the dead body itself. It defiles the crib and the baby, and we reckon impurity as we normally do. Anything that touches the shoe is a “father of impurity” and anything that touches it has first degree impurity and so on.