Nanomaterials get their most striking properties not from a different chemistry but from size itself. This simulator models quantum confinement in a semiconductor nanocrystal (a "quantum dot"): as the crystal radius shrinks toward the nanoscale, electrons are squeezed into an ever-smaller particle-in-a-box, their allowed energy levels spread further apart, and the effective bandgap widens. The result is a bandgap — and an emitted-light colour — that is tunable by size alone, the same effect manufacturers use to make quantum-dot displays emit precise colours from a single material. Adjust the radius and material to watch the energy ladder, the E(R) curve and the emitted colour respond together.