Answer: Due to its shape
Explanation: Polarity of any substance can be determined by its shape. It can also be determined by its dipole moment.
It has positive charge at its one end and negative charge on the other end.It has one oxygen atom which has negative charge (being electronegative in nature) and has two hydrogen atoms which has positive charge.
Thus electron charge density present on the hydrogen is being pulled by the oxygen mking it polar in nature.