Combine rhubarb and water in a saucepan. Bring to a boil, then reduce heat to medium. Simmer for 10-20 minutes, until the rhubarb is completely broken down.
Use a fine mesh strainer over a bowl to drain the juice from the rhubarb.
OPTIONAL: If you have extra time, you can use layers of cheesecloth set in a colander, or a jelly bag, letting the rhubarb drain for at least 1 hour.
Pour the collected juices into a saucepan and whisk in the sugar. Bring to a boil, then reduce to medium and simmer for about 5 minutes, until the sugar is dissolved and liquid is syrupy.
Transfer to an airtight container, let cool to room temperature, and refrigerate until ready to use.
Notes
Makes about 1 ½ cups.
To make a rhubarb soda/spritzer, pour 2-3 tablespoons rhubarb syrup into a glass. Top with sparkling water or soda, adding ice if desired. More or less syrup can be used according to taste.