I see I'm late to the party but well, my most early project of a ball bouncing around ended up as some sort of fullscreen particle simulator.
Made in Swing, had four particle guns you could choose from (water, snow, fire, seed) that interacted with each other, had some gravity applied to them that you could adjust with + and -, and gravity direction with the arrow keys.
All stuff bounces around a lot. You could stack snow into piles and burn it down with fire, water + seed would make a tree that could also be deleted with fire. Fire also deletes water.
It was quite elaborate, even has a settings menu with a funny option called "transparent background". Iirc it took an image of your desktop with awt robot right before starting to fake the effect
Eventually a classmate told me "hey that looks like powedertoy". So I found out what powdertoy was, messed around with it for like a week straight before going onto the next project This was like, dunno 3 or something years ago.
Anyhow I still got the thing (which got me slightly confused which tense to use while writing this lol), here's a download if any of you have too much time to spare.