Today's question: Why does drinking water get bubbles in it after it is left out for a few hours?

Plain Talk answer: Gases from the air, nitrogen and oxygen mostly, can dissolve in water (just like sugar dissolves in water). The amount of air in the water is different at different temperatures, however. Cold water can hold more dissolved air in it than warm water. A glass of water left at room temperature warms up after a while, and the dissolved air is forced out of the water into bubbles. This relieves the condition that is called supersaturation, which is what happens when the water contains more air than can be dissolved in the water.

For more information on this and many other water-related topics, check out Plain Talk About Drinking Water by Dr. James M. Symons.