They fit halfway into the ear canal, and they are able to create an acoustic seal when inserted properly. They last for about a month and then the battery expires, and you dispose of them and buy new ones. Standard batteries for hearing aids will typically last between five and fourteen days, depending on the type of battery and how often the hearing aid is used. You can tell that it is time to change the batteries in your hearing aid when you notice that the sounds are distorted or you have to turn the volume up more than usual.