Maxwell demons are small scale constructions that seem to violate the second law of thermodynamics. One of the prominent examples is the ratchet and pawl construction, presented in the Feynmann Lectures of Physics. We will show that this model is unnecessarily complicated. We propose a simplified model, consisting of units involving only hard core interactions, that can be studied in great detail by molecular dynamics and for which an exact analytic treatment is possible in the limit of dilute gases. In particular we calculate the speed and heat conductivity of the Brownian motor that appears when two reservoirs at different temperature are considered. Refs. C. Van den Broeck, R. Kawai and P. Meurs,``Microscopic Analysis of a Thermal Brownian Motor", Phys. Rev. Lett. 93, 090601 (2004). P. Meurs, C. Van den Broeck and A. Garcia, ``Rectifying Thermal Fluctuations in Ideal Gases", Phys. Rev. E 70, 051109 (2004). C. Van den Broeck, P. Meurs and R. Kawai, `` From Maxwell Demon to Brownian Motor", New Journal of Physics, to appear 2005 (free internet access).