Add water, sugar, vinegar, and ketchup in a small saucepan and heat over medium heat. Whisk until the sugar is melted.
In a small bowl, whisk together cornstarch and 2 tbsp water.
Once the mixture in the pot starts to boil, slowly add the cornstarch slurry while whisking continuously. Whisk until the sauce is thickened to your liking.
Remove from heat and serve.
