A flute player tunes her instrument when the air (and the flute) is cold. As she plays, the flute and the air inside it warm up. Both the changing speed of sound in the air inside and the thermal expansion of the flute affect the frequency of the sound wave produced by the flute. After some time, she finds that her playing is "sharp" the frequencies are too high. Which change produced this effect: the warming of the air or the warming of the body of the flute?