It goes something like this:

A woman goes to the funeral of a family friend and meets a guy she instantly likes. She fails to take his number though and moves on.
6 weeks pass.
She gets away with murdering her own sister.
Why did she kill her sister?

---------------------------------------------------------------------------------------------

My sister once did that test with me. Mind you, I purposfully did not give the "correct" answer, which is that the woman wanted to see the guy again I do think you can arrive at that answer without being a psychopath.