A carbonated drink, such as Coca-Cola or Mountain Dew, is said to be "flat" when most or all of the carbonation has escaped into the air. Since most soda drinks are composed primarily of corn syrup and artificial flavors, they are usually very nasty once the carbonation (and coldness) are no longer covering up the yucky ingredients.

Some people claim to like flat soda; these people are few, far between, and eccentric. The only soda drink that tastes good flat is seltzer water, for obvious reasons. Flat Coca-Cola makes a very good toilet cleaner -- pour it in, brush, and flush. Don't buy Coke just to pour it in the toilet (it's much more expensive than generic toilet cleaners) but if you have flat Coke lying around, this is a good way to get rid of it.