If she asks what your favorite (fill in the blank) is, then you are supposed to have a clear favorite.
Ok here is the dirt on this rule. While this rule holds true for most things, this particular rule is referencing lingerie. Leading up to and just after our wedding, my wife spend a good amount of money buying sexy lingerie. Now she asks which one is my favorite. I like them all and I really don't have a favorite, but a line like, "I love them all why don't you pick the one that you think is sexiest and wear that," does not fly.