We analyze dental (tooth-jaw) organ. This organ is regarded as a natural adaptive system. We develop a thesis of optimality of this biological structure. In order to describe the adaptability, it is proposed to use a unified approach based on formulation of a problem of movable boundary. We make an attempt to estimate a position of the phase boundary `enamel-dentine-pulp’. In order to do this, a unidirectional strength of physiologically healthy tooth is postulated, and the known data on its morphology are compared with the results of calculation of the stressed state under loading during chewing. When considering the model, we consider a piecewise heterogeneous elastic body, which include a dentine kernel, an enamel layer of variable thickness, and a massif of periodontal tissue bordering with the roots. We found out that the initial distribution of stresses keeps the same till the enamel cover wear will reach a definite value. Under local affection (caries), a concentration of equivalent stresses appears, which promotes tooth destruction. Sealing of cavity provides recovering of the initial stressed state.