...the thing to do is to build yourself a catpult. So that's what 8yo and I did, with help from power-tool-dude (Dad).
Too bad I won't post videos of my kids here, because the first attempts were priceless.
1) We didn't have anything to stop the catapult arm, ... except the concrete ... so we launched the water balloon right into the concrete. Well, actually, the bowl that holds the water balloon tried to keep going, past the concrete, but couldn't, so some parts went right, and some went left. We got a new bowl.
2) The springs that provide the force to swing the arm around were pulling in a direction to KEEP the arm in the firing position, so 8yo let go to fire and .... nothing happened.
3) The springs were in the right position, but didn't pull hard enough, so the ball "doinked." 8yo cried, "Aaaw, that sucked!"
4) After a few more tries, we got it go about 30 feet. 8yo's verdict: "That was pretty good!"