Answer:
There are two main types of ear plugs, with distinct consequences for the Halacha.
Foam ear plugs spring back more or less to their original shape after use and are permitted to use on Shabbos and Yom Tov.
However, wax or silicone ones must be shaped from a round ball into a usable form that will fit in the ear. The act of creating a functional shape is potentially problematic, and should be avoided.
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