title_body: What factors affect the carbonation level of beer?
upvoted_answer: There are two main ways to carbonate beer. 
  
  Force carbonation - This is where CO2 is forced into the fermented beer. Because of the pressure (and the temperature at which it is done) the CO2 will dissolve into the beer solution. The fizz that happens when you open a beer is the CO2 coming out of solution. (Technically you can re-carbonate flat beer!)
  
  Natural carbonation - All beer has SOME yeast present. Home-brewers that bottle will actually pour a little bit of sugar into the racked beer before bottling. The yeast that is present will re-activate and eat the sugar. One of the bi-products is CO2. Because the beer is bottled and the CO2 has nowhere to go, it will dissolve into solution. This is why you will often see little yeast cakes in the bottom of home-brewer's bottles.
  
  There are some beers that used forced-nitrogen to nitrogenate (if that is the appropriate word) their beers. 
  
  TL;DR - Temperature, methods of carbonating, storing beer can all affect carbonation.