Amazing Alex The Backyard level 2-1, called “Fill in the Blanks”, is a great example of counter-intuition. While you may want to fill in absolutely every gap with a horizontal pipe in the top row, an elbow pipe in the bottom-left, and a horizontal pipe in the bottom row, this will miss the second star. Instead, neglect the top pipe and let the 8-ball fall through that star.
If you have a different strategy for completing this level feel free to leave a comment below.