Condensation.

Bubbles rarely form on the outside of a glass. Condensation is "the the process of changing from a gaseous to a liquid or solid state." In this case gaseous water condenses into small globules of liquid water on the outside of a glass. Small globules of liquid are called drops or droplets, not bubbles. Drops of water rarely form on the inside of a glass of water, or if they do how could you tell? Bubbles are " hollow globules of gas." They can form inside a glass of water because the gas 'air' comes out of solution from the liquid. A globule of air, a bubble, could form inside a globule of water, a drop, on the outside of a glass, the drop mighta formed from condensation but the bubble formed from dissolution, or possibly a spilled drop surrounding some gas. Sometimes a distinct globule of liquid can form and be suspended in another liquid. Shake up a bottle of unemulsified vinegar and olive oil, small globules of oil will persist for a short time in the vinegar, some folks call the oil globules 'bubbles' , but it's olive oil not olive gass, the globules are droplets. Oversimplifying, droplets form from condensation, bubbles form from expansion. Do not gobble your globules, you will get bubbles in the tummy, possibly escalating to gas, possibly to something more disgustingly liquid. BTW, the answer to the original question is, paraphrasing my self, bubbles form on the inner sides of "a glass of water because the gas 'air' comes out of solution from the liquid" 'water.' BTBTW, see question 26 in this category, where I answered the same question, with, however, much unneeded verbosity, expansion, and hot air. I shoulda condensed it and not got so gassy. I think it's the same question, it could be read as asking why the glass of water forms bubbles. Of course neither the glass nor the water forms bubbles, the gas in the water inside the glass forms bubbles. I think I prefer the phrasing of this version, 'specially since it specifies 'tap water,' and with the minor change of 'sides' to 'inside' of a glass. BTBTBTW- the preferable way to correct what one feels is an unfair rating, is to correct the answer, or question the rating there, not by uprating your own answer.

See this link: http://www.sciam.com/article.cfm?id=why-do-bubbles-form-if-a