The first player chooses a positive even number that is less than 50, and crosses it out on the 100 square grid.
The second player chooses a number to cross out. The number must be a factor or multiple of the previous number.
Players continue to take it in turns to cross out numbers, at each stage choosing a number that is a factor or multiple of the number just crossed out by the other player.
The first person who is unable to cross out a number loses.
For one player, or as a class competition, follow the same rules and see who can make the longest chain of numbers.
Here is an interactivity for this game on NRICH where you can download a version to your own tablet or smartphone.