In sensory evaluation of mechanical properties, the gauged object and the tissues in contact (i.e. the hand, mouth, etc.) form a system that can be described by an in-line array of two models, the object's and the tissue's. The hypothetical rheological properties of such combined systems were studied qualitatively using simple phenomenological models with and without failure criteria. It is demonstrated that the mechanical stimulus reaching the human sensory system can be different in both kind and magnitude from the signal produced by testing machines irrespectively of the test geometry. It is also shown how the deformation rate can affect the rheological character of the stimulus and how damage to the sensory system (particularly the teeth) can occur if the deformation rate is not controlled.