Why is the 7th row always flat? Drag the bottom balls left and right. The rows above will become uneven - except for the 7th row which is always flat. Why? This is not a programming trick. The same would happen if you stacked wine bottles 7 rows tall IRL. So what is the secret of the 7th row? (or more for stacks where the bottom row contains n balls, what is the secret of the (2n-1) row?
Mathologer: https://www.youtube.com/watch?v=cmdOmOCfo90
