  We present a simple, and physically motivated, coarse-grained model of a
lipid bilayer, suited for micron scale computer simulations. Each ~25 nm^2
patch of bilayer is represented by a spherical particle. Mimicking forces of
hydrophobic association, multi-particle interactions suppress the exposure of
each sphere's equator to its implicit solvent surroundings. The requirement of
high equatorial density stabilizes two-dimensional structures without
necessitating crystalline order, allowing us to match both the elasticity and
fluidity of natural lipid membranes. We illustrate the model's versatility and
realism by characterizing membrane response to a prodding nanorod.
