"I try to make puzzles range all the way from easy to hard, and to leave many open at once"

Graham Nelson is suggesting that when creating puzzles, it is important to have a series of trouble levels. This allows people of all skill levels to be able to delight in the puzzles. By having a series of problem levels, it allows individuals to begin with the easier puzzles and work their method up to the more difficult ones. Additionally, by leaving numerous puzzles open simultaneously, it allows people to work on numerous puzzles at the same time, which can be beneficial for those who are trying to find a difficulty. This also enables individuals to switch between puzzles if they become stuck on one. Overall, Graham Nelson is suggesting that having a series of problem levels and leaving many puzzles open at the same time is useful for those who are aiming to enjoy puzzles.