The lower part of the tile is a pool that obeys gravity: tilt the device and the surface line tilts and sloshes, modelled as a damped spring plus three traveling ripple modes excited by every tilt change. On desktop the pointer x drives the tilt and drag speed stirs it. Dark liquid with a glowing meniscus, fresnel edge, faint caustic shimmer, and a few overhead light dots reflected into the water. Left alone it sways and throws the occasional deliberate slosh.