Re: Choose a question (and answer) on car numbers
(Leaving aside that car registration numbers can be a bit short)
"What was the registration of your first car?"
Okay, fair enough.
"What was the registration of your previous car?"
What happens if you change cars one or more times after choosing that question and setting the answer?
You set it up and enter AB12 CDE as your answer - then n years down the line, you have to resort to answering that question. You think back, and remember that the registration of your previous car (to the one you now have) is VW12XYZ.
"What was the registration of your red car?"
Even if you select a colour for which you have only had one car, the n years later problem still applies - between setting that question and answer and the arbitrary point in the future when you need to answer that question, you may have had more.
"What was the registration of your father's car?"
Which one? (Car, not father!) My step dad has had quite a few in the 40+ years I've known him!
"Would work for quite a few people."
I see flaws. :)
The advice I generally give to people is to treat "secret questions" as password prompts, and enter a sensible password instead - especially on sites that have replaced passwords with secret questions (HSBC, I'm looking at you - the use of 2FA does not make this acceptable). However, since it's likely that (because they aren't passwords) many sites won't salt/hash the answers, this makes it even more important to ensure that password is unique. (So use a password manager such as KeePass)