little swimmers
boy sperm (Y) swim faster than girl sperm (X)
but girl sperm live longer (they pace themselves)
Boys sperm are like sprinters, girl sperm are like marathon runners.
That's why if a couple want to have a boy they wait until after the woman has ovulated to have sex, if thy want to have a girl, they have sex the day before the woman ovulates.
Boys want to hurry up to get the prize whereas girls take their time and are willing to wait and linger to get the prize.