Mix together water and sugar in a heavy sauce pan, along with whatever you want to flavor your soda. Put on stove on high heat. Heat to boiling, stirring occasionally. IMMEDIATELY remove from heat. Note: Really, watch closely so it doesn't boil over. It boils over easily.
Turn stove down to simmer (lowest setting), and return pan to heat. Make sure that you've waited long enough for the heat to actually go down to a lower setting, so you aren't in danger of boiling the syrup over again. Simmer for 15-20 minutes, stirring occasionally.
Add 1/4 tsp cream of tartar, and let cool.
Pour into a container and save to use! Add a small amount to the bottom of a glass, and top it off with carbonated water. We used about 3 tablespoons soda syrup for our 12 oz. bottle of soda, but you can use whatever tastes right.
