In a heat engine, heat can diffuse between the hot reservoir and the cold reservoir and in Chapter 10 we showed that this takes place on a timescale which scales with the square of the linear size of the system (see Example 10.4). The mechanical timescale of an engine typically scales simply with the linear size of the engine. Explain why this means that heat engines don't work on very small scales. [This is one why reason why the ``engines'' powering biological systems, which have to be extremely small, are not heat engines. Instead, useful energy is extracted directly from chemical bonds. Heat engines also often run on chemical fuel but use the fuel to heat one of the reservoirs and then extract work from the temperature difference thereby generated.]