Definition of a Hearing Aid

I love this kind of occupation-specific humor, as seen on my audiologist's office wall:

A hearing aid is an ultra-miniature electro-acoustical device that is always too large.

It must amplify sounds one million times, but bring in no noise.

It must operate without failure in a sea of perspiration, a storm of talcum powder, or a cave of wax.

It is a product which one puts off purchasing for ten years, but then cannot do without for 30 minutes of servicing.