Figure 3.

The process of extracting work from a system, thought of by Szilard: in (a), there is a single molecule of a fluid inside a box with energy Q. If one knows in which half of the box the molecule is (i.e., a single bit of information about its position), a piston can be inserted by halving the box (b) and from the fluid expansion, work ((c,d)) $W=Q$ can be extracted from the system while it returns to its initial state.