Rehydrate the water kefir grains if necessary (see notes).
Fill the half gallon jar with room-temperature non-chlorinated water. Be sure to leave enough room at the top to add the sugar and the water kefir grains.
Stir in the sugar until it is dissolved.
Add the kefir grains to the jar, along with a couple ounces of water kefir starter if you have it from a previous batch. (Adding the water kefir starter is not required but does help the fermentation get going quicker).
Cover the jar of water kefir with a cheesecloth and secure it with a rubber band or string. Date the water kefir jar so that you remember when you fed it.
Let the water kefir ferment at room temperature for 12-48 hours.
When it is done fermenting, strain out the water kefir grains with a fine mesh sieve lined with cheesecloth. Reserve the grains and a small amount of liquid to start a new batch.
You can refrigerate the fermented water kefir and enjoy it as is, or you can do a second ferment in order to create a delicious, probiotic-rich soda.
Notes
Sometimes, kefir grains will arrive dehydrated. Follow the directions on the package or in the post to rehydrate the grains before proceeding with this recipe.
In this recipe, I use 1/2 cup of kefir grains, so I add 1/2 cup of sugar to feed them. If you work with fewer grains, you can feed them less in a one-to-one ratio and use a smaller jar until they grow and multiply.
Water kefir grains grow and multiply rapidly. You can gift some to a friend, or you can start multiple batches with several jars.
Always remember to date your jars to keep track of the process.