Hair cells are the receptor cells that enable the transduction of mechanical stimuli into the nervous system of hearing and balance organs of vertebrates. The name of these cells owes to the specialized organelle located at their tip—the hair bundle. The organelle's complex structure and operation, which is enhanced by an active process, are crucial for an organ's sensory function. In my talk I will tell about a few open physical problems related to the development and operation of the hair bundle.