Hard Disk Gas models a two-dimensional gas by extending the collision model to multiple particles. Particles are initialized with a speed v=1 in a random direction and move with constant velocity until a collision event is detected. Conservation of momentum and energy are applied and the simulation is resumed until the next collision event is detected. Although collision detection is not the most efficient computation algorithm for this problem, it is straightforward, accurate and fast-enough for small numbers of particles to show the physics that gives rise to a simple model of an inert monatomic gas.