As hair bundles move, viscous friction between stereocilia and the surrounding liquid poses a physical challenge to the ear’s high sensitivity and sharp frequency selectivity. This letter proposes that some of that energy is used for frequency-selective sound amplification, through fluid–structure interaction between the liquid within the hair bundle and the stereocilia. A dynamic model is proposed to simulate hair bundles in a viscous environment, to see what large and small scale insights could be gained. Finite-element analysis, a submodel of hydrodynamic forces, stochastic simulation, and models of interferometric measurement all aimed to simulate both a hair bundle in natural conditions and what might be observed in an experiment involving it. Forces between stereocilia are estimated, and the results suggest that the closeness of stereocilia reduces drag between them, supporting a sliding but not a squeezing mode. Tip links may couple mechanotransduction to this low-friction sliding mode, with motion between neighboring stereocilia of less than 1nm when the hair bundle moves the larger distance [O(10nm)]needed to stimulate its channels.