One strategy for Amazing Alex The Backyard level 2-31, called “Juggling Time”, is to place the pail just under the left tennis ball with a spring underneath. This combination should get you the star on the left side. Place the scale so that the tennis ball is above the left arm. Place the bowling ball a bit above the tennis ball to catapult it into the top star. The objective is to simply get the tennis balls above the ropes.
If you have a different strategy for completing this level feel free to leave a comment below.