[jtheory]

How many common "first instruments" are there for kids? Maybe 5? There's a pretty good chance you'll get it right without doing any research whatsoever.

It's not quite as bad as asking "what species was your first pet?" but not much better.

This is the real problem with security questions; the answer space is often so very small, and can be narrowed down even further with a little research.

Questions like "what was the first name of (your maternal grandmother, your first best friend, etc.)" are very common -- well, there are stats on most popular first names of given generations in different places. If you know what country the person is in, you can make a good guess at these.