Why does blowing over a bottle make a note ?

Resonance in acoustics occurs when some mass-spring combination is
supplied with energy. Many musical instruments rely on air resonance
to improve their sonority. If you blow across the mouth of a bottle you
can often get a note. The bottle behaves as a Helmholtz resonator. The
main volume of air inside the bottle is analogous to a spring, whilst
the “plug” of air in the neck acts as an attached mass. The resonant
frequency is roughly given by:

f = { c sqrt (S/LV) } / 2pi

c is velocity of sound
S is the surface area of the neck opening
V is bottle volume
L is the effective length of the neck ie the actual length plus ends
correction. Ends correction ~ 1.5 times radius of neck opening

Example: A 75 cl (7.5E-4 m^3) wine bottle with neck diameter 19 mm,
bottle neck length 8 cm, air temp = 20 degC
calculated resonance = 109Hz (actual resonance was 105Hz)

Helmholtz resonators are sometimes employed as a means of passive noise
control in air conditioning ducts. They may also be hidden in the wall
design of auditoria and offices in order to improve the acoustics.