system
Given the following post, respond as if you're commenting on Reddit:
human
Does water have a high heat capacity because it is a polar molecule? Do all polar molecules have high heat capacity?
gpt
The heat capacity of water is high because it has a great many numbers of *degrees of freedom*.

Water is a bent molecule.  That means it has 9 *possible* degrees of freedom: 3 translation degrees (x,y,z), 3 vibration degrees, and 3 rotation degrees.  Moreover, because water is also a polar molecule, and because individual water molecules are capable of interacting with each other via hydrogen bonds, these 9 degrees of freedom are relevant because the water molecules are capable of transferring energy between each other and into those 9 degrees of freedom.

Additionally, and this is what makes water really special, all 9 of those degrees of freedom correspond, on a quantum level, to relevant energies that you'll encounter at STP.  That is to say, there are degrees of freedom that are actually relevant at the temperatures (energies) we're actually interested in!  If you were to take, say, H_*2*_, which is a straight (not bent) diatomic gas, you find at the quantum level that yes, strictly speaking it may have three degrees of rotational freedom, but the rotation *along* the axis - spinning like a drill - is not relevant because the energy levels are so huge that it's not possible to transfer energy into that degree.  It turns out, that in water all 9 of those degrees correspond to energies you're likely to find at STP.

If you calculate out the thermodynamic temperature of water:

> T = 1 / (δS/δE)

...you find more degrees of freedom mean that the entropy (S) increases much more for water given an infinitesimal of energy.  A hand-waving, back-of-the-envelope explanation would be to say that there are more energy sinks available to absorb thermal energy before the temperature increases.

Now on top of all that, all those degrees of freedom also happen to be attached to a very "light" molecule.  Sure you can find more degrees of freedom in some polymer, with hundreds of atoms in a chain, but those damn things weigh a ton, and every atom you add to the molecule just increases it's molar mass, which renders intermolecular forces like hydrogen bonding less relevant while at the same time reducing C_*M*_ by increasing said mass.  As for C_*V*_, water also does a hell of a job keeping it's volume pretty low by existing in a liquid state at much higher temperatures than what one would expect for exactly those reasons: The (relevant) intermolecular forces. 
 Water is just about the lightest compound one can think of that combines all these properties.