What is a cause of polarity in water molecules?
the polarity of the O-H bonds and the asymmetry of the molecule.
the O-H bond has a dipole moment of ~1.4, a strongly polar bond creating partial charges (neg on oxygen, positive on hydrogen).
The lone pairs on the oxygen molecule create a "bent" shape (the valence electron pairs repel the O-H bonds, folding them together at an angle of ~104.5). This creates a distinct region of negative charge around the oxygen atom, and a pair of distinctly positive regions around each hydrogen atom (on same side of molecule).
The greater the difference in electronegativity and the greater the asymmetry, the more polar a molecule will be.