"We're counting that like wanting and not wanting a horse.  You've got to want one more than the other, in terms of which one you'll trade for the other one, or else be indifferent, and that's what we're calling a preference.  So if you trade it one way and then trade it back the other way, we'll say your preference reversed over time."

"We're counting that like wanting and not wanting a horse.  What we're calling a preference is shown by the act of trading one of these things for the other.  So if you trade it one way and then trade it back the other way, we'll say your preference reversed over time."