Mikvaot, Chapter Ten, Mishnah Two
1) A bolster and a cushion of leatherit is necessary that the water enter inside them.
2) A round cushion or a ball or a bootmaker’s last or an amulet or a phylactery, it is not necessary that the water enter inside them.
3) This is the general rule: any article the filling of which is not usually taken out and put in may be immersed unopened.
Section one: This section lists coverings which one regularly puts stuff in and takes stuff out of. Therefore, the insides must be immersed as well as the outsides.
Section two: In contrast, these items have something inside of them which is not customarily removed. Therefore, they do not need to be opened up in order to be immersed.
Section three: This is the general rule that explains the distinction between the items in sections one and two.