Combine sugar, root beer extract and 1 gallon warm water in 5 gallon tall cooler. Stir until sugar is dissolved. Add 3 pounds block dry ice; this will cause mixture to bubble; add 2 gallons water and have lid ready to put on container.
do not put it on tight. let bubble for about 1/2 hour and add rest of water. Root beer will bubble about 2 hours. When bubbling has stopped, root beer is ready to drink.
Put in gallon plastic jugs and cap. Will keep in refrigerator for some time.