Cloth simulation
Fabric as a mesh of point masses held together by distance constraints, relaxed every frame until the cloth hangs right.
Xavier Provot described the model games still lean on back in 1995: a grid of particles joined by springs or stiff distance constraints — structural, shear and bend links — stepped with Verlet integration and iterated a few passes per frame. Cheap, stable, and convincing enough for a cape or a flag.
The expensive part isn't the sheet itself but stopping it passing through itself and the world. Self-collision needs spatial indexing, and a fold that resolves the wrong way looks wrong instantly. Position-based dynamics (Müller et al., 2007) fixed much of the stability problem by correcting positions directly instead of forces.