= Solution
A nonlinear triatomic molecule has $3N-6=3$ vibrational <normal modes>. For water they are the symmetric O–H stretch, the H–O–H bend, and the antisymmetric O–H stretch, shown above. In the symmetric stretch both O–H lengths increase together; in the antisymmetric stretch one increases while the other decreases; the bend predominantly changes the bond angle. All three change a component of the <electric dipole moment> and are infrared-active. Every arrow reverses half a period later; motion of oxygen maintains the molecular <centre of mass>.
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