Frogs, and related amphibians, are adapted to live in both air and water, and so good hearing in both mediums is required. The structure of the frog ear can therefore provide a useful perspective on how sound is sensed and the physical principles involved in hearing. This broad survey of the literature highlights two noteworthy aspects of the frog ear and brings them together into a single framework. First, the frog ear contains an arrangement of sensing cells which is difficult to understand: although they are meant to detect sound, the cells are hidden away in recesses and further shielded from incoming sound by a number of ‘short-circuits’ in the vibratory pathway. Second, there is the operculum, a moveable plate that fits into the oval window adjacent to the stapes and whose function remains controversial. Both these challenging features can be understood by noting that all sounds carry both pressure and displacement components, and that form and function can be matched by focusing on the pressure component, which to date has been largely overlooked. This paper proposes that the hair cells at the core of the system respond more sensitively to pressure than to displacement. Building on this property, the piston-like operculum, operated by the opercularis muscle, is put forward as a mechanism for adjusting the static hydraulic pressure within the otic capsule, in this way controlling the global sensitivity, or gain, of the sensing cells within. Both these hypotheses have wider implications for understanding hearing in vertebrates.