- Click to place a body.
- Scroll or use arrow keys to change mass.
- Drag to give it a velocity vector.
- Press space to create a cloud.
- Press K to fill the screen.
- Middle-click or shift-click and drag to move the viewport.
- Shift-scroll to zoom.
- Press C to track the most massive object.
- Press D to delete all bodies not on the screen.
- Press Q/A to increase/decrease the gravity constant.
- Press P to pause/unpause.

Disclaimer: Parts of the JavaScript used to make this app were borrowed
from other scripts that I found while searching for help with the math.
I have no one to credit for this. But if any of this looks familiar it's
probably because I borrowed other people's ideas.