In a small saucepan, combine the water and demerara sugar. Place the saucepan over medium-high heat. Stir continuously until the sugar has completely dissolved.
Bring the mixture to a gentle boil, then immediately reduce the heat to a low simmer. Let it simmer gently for 2 minutes. Remove the saucepan from the heat and let the syrup cool completely.
Pour the syrup into a clean glass jar or bottle. Store it in the refrigerator for up to 3 weeks.