easy
step 1: take the CoD out of its case and replace it with a worthless CD
step 2: give the case to your parents and say this: "mother, if you don't think I am proper enough and grown enough to play this historical game then I will allow you to take the game back" now from here, 2 things will happen:
A: she thinks your grown up enough to keep it and you get to play CoD :D
B: she takes the game to return it (dont forget you put a worthless CD in so you still get to play CoD :D)
...or you could just grovel on your knees to keep it...?