Earhooks are curved, hollow "hooks" that screw onto to the hearing aid body. Earhooks serve multiple purposes. One important purpose is to hold the hearing aid on the ear! The earhook is a removeable extension of the hearing aid that "hooks" the top of the ear to hold the hearing aid in place (push "View On Ear" button). The earhook also provides a connection between the output of the hearing aid's speaker and flexible tubing that further directs sound into the ear canal.
The earhook can also influence sound by containing a small damper (push "View Damper" button). Because earhooks are typically clear, you can usually see the damper, which looks like a small, white piece of cotton. Dampers can be made out of a variety of porous materials (like lamb's wool) that alter the sound waves that pass through them. It is important not to push the damper out when cleaning an earhook, and to make sure a replacement earhook has the same type of damper.