My Rube Goldberg project was challenging because it was hard to reset all the dominoes, books, and baseball.  The balloon popped a lot because the placement of the balloon was bad because I put it too close to the thumb tack. When I was trying to put it in place, the toy car with a thumb tack never worked because I had to find a sharper thumb tack and make the balloon closer to the thumb tack but not too close.