give an inch, they try and take a mile!

Does anyone else have a "bargainer" in their family? Mine is of the 3 1/2 foot tall variety and can be found as close to me as a shadow when she wants something. When she is given something - tonight was "one story before bed" - she then says, "Can I have (what's given + 1)?" My response of late has been, "Nope. And now you can't even have one! We don't bargain." I've reduced desert like that before and even minutes for an activity. Usually my method is halving whatever she could have had before. For example with cookies - when bargaining for a third, two cookies get reduced to one. You'd think some of the really good stuff being cut in half would have an impact! Errrr....