Potting Drill.
This is a very difficult exercise that only the best of players will be able to complete.
Pot the balls in succession, red-black-red-black, starting with the red closest to the black.
The reds will obstruct each other so that they must be potted in sequence, alternating across the table.
If you can pot four reds and blacks you are doing quite well, if you can pot all nine reds you are well on your way to becoming a champion.