The hippocampal-entorhinal system builds the brain's cognitive map of space using two complementary cell types. A hippocampal place cell fires in one localised patch of an environment, like a single labelled landmark; an entorhinal grid cell instead fires everywhere the animal crosses the nodes of a regular hexagonal lattice, acting like graph paper laid over the world. This simulator drives a virtual animal through an open arena — by random-walk autopilot or by your own dragging — and computes each cell's instantaneous firing rate live from its position, painting the theoretical receptive field onto the floor so you can see exactly where each cell type does and doesn't respond. Switch cell types and adjust field width, grid spacing and orientation to see how the underlying geometry changes what gets encoded.