Cannon Ball sees you shooting a spear or other similar projectile straight up into the air, in an attempt to eliminate increasingly smaller balls jumping around the room you're stuck in. The good news is, you don't have to be all that accurate. If the ball bumps into the trail your spear leaves behind it, that's considered a hit. Similar to Asteroids, if you hit a ball it splits into two. Eventually they get small enough that the hit eliminates it from the playfield. Do that to all balls and you are rewarded with the next stage, same as the previous but with an extra ball to eliminate.

Oh, and of course, you don't want to get smashed.