Identical balls are placed on the plane so that their centers form the nodes of a square grid. The gaps between the closest balls are the same and very small compared to their radius. One of these initially quiescent balls was given a velocity v at an angle α to the side of the square cell. What will be the further movement of the balls, if all the blows are elastic? Consider qualitatively the case of a lattice with a cell in the shape of a regular triangle.