Use the door-in-the-face technique. Ask for something big, they'll reject it, then ask for something smaller, the thing you really wanted.
At twelve it may work like this: "Mom, Dad, Can I have a car?" "No." "Well, can i at least get a new bike?" "...Okay. Sure."
And at sixteen it may work like "Mom, Dad, can you buy me a car?" "No. Get a job." "Fine, fair enough, but can I at least borrow your car?" "...*hands you the keys* Don't speed."
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Use the door-in-the-face technique. Ask for something big, they'll reject it, then ask for something smaller, the thing you really wanted.
At twelve it may work like this: "Mom, Dad, Can I have a car?" "No." "Well, can i at least get a new bike?" "...Okay. Sure."
And at sixteen it may work like "Mom, Dad, can you buy me a car?" "No. Get a job." "Fine, fair enough, but can I at least borrow your car?" "...*hands you the keys* Don't speed."
Tell them that it's because you want to make sure you're not (insert something they don't like, for example gay, communist, etc.).