Math Games to Amuse and Confound

Here I am, solidly in my fifth decade of life and it has never occurred to me to play, let alone invent, a math game (in fact, I started to write fourth decade, then corrected myself and still had to doublecheck with my husband. That'll tell you something about the distribution of mathematical abilities in this house). My children, however, have inherited math genes from their dad and so driving home from school we have conversations like this:

Ben: Think of a number less than 100 but an even ...