Assume you have an infinite checkerboard, with an infinite horizontal line. All the squares beneath the horizontal line have a pawn sitting on top of them - there are infinite pawns.
A legal move for two pawns A, B is as follows. The two pawns must be adjacent. Then A can jump over B, to the spot after B (which must be empty prior to the move).
Show that under no circumstances you can reach the row above the horizontal line