Answer:
There are two reasons. One of them is that the water molecule is “bent”. It is rotationally asymmetric, meaning that it can “store” more energy in the rotational degrees of freedom than other, symmetric molecules (like diatomic molecules, or CO 2 for example).
The other and more important reason are the hydrogen bonds. When a hydrogen atoms binds with a more electronegative atom (such as oxygen in the case of water), its electron has a higher probability to be found in the vicinity of the oxygen atom, leaving the hydrogen nucleus partly “exposed”. This partial charge on the hydrogen will create a very weak bond with the partly negatively charged oxygen atom of a neighbouring water molecule (remember, the hydrogens’ electrons prefer to be around the oxygen atom instead, making it a bit negatively charged). This electrostatic interaction, or bond, between neighbouring water molecules is the famous hydrogen bond.
Even though each hydrogen bond by itself is weak compared to the bonds in the molecules themselves, there are many neighbours around each water molecule, which means that water can store a significant amount of energy in these bonds. As you add heat to water, instead of speeding up the molecules (increasing their kinetic energy and thus temperature) the hydrogen bonds get broken up, soaking in the additional heat without much change in temperature.
Step-by-step explanation: