A biologically necessary property of thermosensory TRP channels (e.g., heat-activated TRPV1, cold-activated TRPM8) is high sensitivity to temperature change. ΔHo values for channel opening are typically 50-100 kcal/mol. The molecular source of such unusually high enthalpies has posed a persistent mystery in this field. We postulate here that these arise from a large molar heat capacity increase accompanying TRP opening, ΔCP ∼ 5 kcal/mol-K. (This idea is strictly analogous to the huge enthalpies of heat-driven and cold-driven protein unfolding, which reflect exposure of nonpolar groups to water, wherein ΔCP ∼ +15 cal/mol-K per CH2 group). Both ΔHo and ΔSo then become temperature-dependent, and the channel's opening equilibrium constant, K, becomes necessarily non-monotonic (“U-shaped”) with temperature:lnK(T) = lnK(To) + (ΔHo(To)/To - ΔCP) (1-To/T)/R - (ΔCP/R)ln(To/T)where To is an arbitrary reference temperature.The position of the “U” on the T-axis is determined by ΔHo(To), and the steepness of its rising arms by ΔCP. This behavior is a necessary, model-independent consequence of the First and Second Laws of thermodynamics; the only question is whether TRP opening exposes enough grease to give experimental temperature-sensitivity. Simple arithmetic shows that only ∼20 hydrophobic residues in each subunit need expose themselves to give ΔHo values observed in the physiological range. Moreover, the position of the U-shaped K-vs-T curve on the temperature axis determines whether a TRP channel would display heat- or cold-activation. According to this picture, we conclude that (1) heat-sensing and cold-sensing arise from the same type of conformational change, (2) all heat-sensing TRPs are also cold-sensing and vice versa (although both rising arms of the “U” may not be experimentally accessible in a given TRP), and (3) attempts to identify “temperature-sensor” domain differences between heat- and cold-activated TRPs are exercises in futility.