Mikvaot, Chapter Ten, Mishnah Three
This mishnah lists things that don’t require the water of the mikveh to enter into them in order to be cleansed.
The following do not require that the water shall enter inside them:
1) Knots [in the clothes] of a poor man,
2) or in tassels,
3) or in the thong of a sandal,
4) or in a head-tefillin if it is fastened tightly,
5) or in an arm-tefillin if it does not move up or down,
6) or in the handles of a water-skin,
7) or in the handles of a wallet.
Section one: The poor man will not undo the knots, therefore water need not enter inside the knot.
Section two: Tassels on the fringes of garments are not generally undone.
Section three: This is the knot that attaches the sandal to attach the forefoot. The straps on the other side would be tied and untied but the thong part would not.
Section four: As we know, straps are attached to the tefillin. If the strap is attached tightly to the head tefillin, it does not need to be loosened to be immersed in the mikveh.
Section five: If the strap on the arm-tefillin is tight enough so that the box doesn’t move up and down on its own, then the water need not go inside the connection between the two. This is a lesser connection then the connection required for the head-tefillin because the arm-tefillin must be adjusted in order to put it on one’s arm.
Sections six and seven: One never removes these handles. Therefore, water need not enter in between the handle and the skin or wallet.