Simplify the following text for a Grade 5 native English-speaking person.
Original: A shoulder rest generally follows the curve of the shoulder ; some shoulder rests are bendable , others are made of sponge-like material , and a few have an extension that hooks further over the shoulder for stability .
Simplified: The purpose of a shoulder rest is to help the player to be comfortable holding the violin , which is held between the chin and the shoulder . Not all players use a shoulder rest .