What does water taste like?

I donnow if it's me, but microwaved warm water is sour...

Air molecules normally dissolve in water and contribute to its taste. Heating (or freezing) the water causes much of this dissolved air to come out of solution as gas and leave the water. That's the origin of the tiny bubbles that form on the walls of a pot of water as you heat it for the first time (they're not steam bubbles; they're air bubbles).

The hotter you heat the water, the larger the fraction of dissolved air you drive out of solution. By superheating water in the microwave, the dissolved air content may be reduced further than it would in a stove-top kettle. The result, as my tastebuds aver, is "flat-tasting" water.