Debug mode expands the HUD with extra pieces of information about the state of the simulation as well as the particle currently under the mouse.

There's a lot of information exposed in debug mode. See the image below:

1. Framerate: this shows how many update cycles the game can do in a single second. 2. Particle count: the number of particles currently in the simulation. 3. Particle type: the type of particle that you're currently hovering over. 4. ctype: the "secondary type" or ctype of the particle you're currently hovering over. 5. temp: the temperature of the particle. 6. life: The life value of the particle. Used mostly to store the amount of frames this particle has yet to exist. 7. tmp: An integer that can be different for every particle. Used for various purposes. 8. index: The particle's index in the underlying list of particles. 9. and 10. are the X and Y coordinate of the particle.