Kelim, Chapter Twenty-Four, Mishnah Eleven
There are three different types of water skins and three different types of shepherds’ wallets:
1) Those that can hold the prescribed quantity are susceptible to midras uncleanness;
2) Those that cannot hold the prescribed quantity are susceptible to corpse uncleanness;
3) And those made of fish skin are free from all uncleanness.
Section one: In 20:1 we learned that for a water skin to be susceptible to midras impurity it must be capable of holding seven kavs and a shepherds’ wallet must be capable of holding five kavs. If they are of this size, then they are also meant to be sat upon.
Section two: If they are smaller than this size, then they are not meant for sitting. They are still vessels, so they are susceptible to corpse uncleanness.
Section three: Fish skin cannot become impure (see 10:1).