Answer
Wind instruments change pitch as they warm up, because the speed of sound is higher in warm air than in cold air.
Work Step by Step
For example, equation 16.18 gives the frequencies of a pipe that is open at both ends, $f_n=\frac{nv}{2L}$. At higher temperature, the speed of sound v increases, which increases the pitch of the notes.
Generally, the instrument is initially cooler than the musician’s breath, so she needs to warm it up before tuning it, or later the instrument will change pitch as it is played.