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How to change a keg
Check to make sure your keg is empty.
You'll know that your keg is empty if you open the faucet of the tap and either very frothy beer comes out or none does.
Look to see if your keg has a co2 line.
Some kegs will have a co2 tank that provides pressure inside the keg to help dispense beer. The gas also helps the beer hold its natural carbonation.