The bubbles form because air (gas) dissolves in water, and it does this more easily when it is cold. So when the water is left out it warms up and less gas can stay dissolved. This causes the small bubbles to form on the side of the glass, which are gases trying to leave the water. The water in the pipes is also under pressure while water sitting out in a glass is not. This also leads to more gas escaping the water.
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