According to an embodiment, a hearing aid is disclosed. The hearing aid includes a first functional unit and a second functional unit. The second functional unit is configured to removably couple with the first functional unit. The coupling provides at least a mechanical connection between the first functional unit and the second functional unit. The first functional unit includes a moveable element that is configured to move between a first position and a second position. In the first position, the moveable element is contained entirely within the first functional unit. In the second position, a part of the moveable element is configured to protrude out of the first functional unit. The second functional unit includes a recess configured to receive the part of the moveable element that protrudes out of the first functional unit such that the first functional unit and the second functional unit are immovably locked with respect to each other in response to the moveable element is received in the recess.