  Two losing games, when alternated in a periodic or random fashion, can
produce a winning game. This paradox occurs in a family of stochastic
processes: if one combines two or more dynamics where a given quantity
decreases, the result can be a dynamic system where this quantity increases.
The paradox could be applied to a number of stochastic systems and has drawn
the attention of researchers from different areas. In this paper we show how
the phenomenon can be used to design Brownian or molecular motors, i.e.,
thermal engines that operate by rectifying fluctuations. We briefly review the
literature on Brownian motors, pointing out that a new thermodynamics of
Brownian motors will be fundamental to the understanding of most processes of
energy transduction in molecular biology.
