Drops on soft surfaces learn the hard way.

A liquid drop sits on a solid surface and slides away when the surface is tilted. Few physical systems seem so mundane or would appear to be governed by such plain, predictable, and long-understood principles. Indeed, Young’s fundamental understanding of the equilibrium gas/liquid/solid contact line (Fig. 1A) has remained intact for over two centuries (1). Nonetheless, static and dynamic contact lines continue to provide surprises (2), yielding qualitatively new insights, motivating new materials, and highlighting the mysteries that can remain overlooked within even the simplest of systems. In PNAS, Style et al. reveal yet another surprise hidden within this classic system: droplets spontaneously migrate along substrates that are chemically homogeneous, but inhomogeneously deformable (3). The Young (or Young–Dupré) equation,