Why does lemonade go flat?

The fizz in soft drinks is carbonation. That is, the addition of CO2 (carbon dioxide) gas. Carbon dioxide readily disperses in an oxygen atmosphere (ours) so simply leaving a soft drink open will cause it to "go flat." However, some containers, usually plastic ones, have small amounts of porous material that allow for a slight leakage of pressure. Many beverage container companies call this "breathing." When the containers age they breathe out the CO2 and breathe in oxygen. Flat again. One of the things that accelerates this breathing is temperature. If you take a beverage container and move it from cold to warm and back again, several times, the change in temperature forces pressure changes that will dramatically accelerate breathing. Flat, flat, flat. Incidentally, there are CO2 dispensers out there (called home fountains) that will add the fizz back to flat drinks or even carbonate anything you want. Check it out.