[newsycaccount]

It doesn't =)

If you have enough heat to make it click forward, you also get random bounces on the prawl which themselves make it go in reverse as much as forward.

http://en.wikipedia.org/wiki/Brownian_ratchet

[aswanson]

What is the entropy of the singularity that produced the big bang? If all the matter and energy in the universe recondensed to that point, what would it's entropy be?