This question was asked by Lucy from Taunton, UK.
You may have noticed that if you leave a glass of water to stand for a few hours, tiny bubbles appear on the inside of the glass. This is because the water that we drink doesn’t contain just water.
Water from the tap is stored under pressure so that it comes out at a reasonable rate. Because of this, any air that’s in the pipes becomes mixed in with the water molecules. The same thing happens to bottles of water; as the water is poured in it mixes with air molecules. We can’t see this because the two substances are mixed together so well.
If the glass of water is left alone, the air and water molecules will gradually begin to separate. The air forms tiny bubbles which fasten onto any nicks, scratches or other miniscule imperfections on the surface of the glass. The longer a glass of water is left, the more air bubbles you’ll see.