Best Sweet and Sour Sauce

In a small saucepan, combine sugar, vinegar, ketchup, and soy sauce. Heat over medium, stirring continuously until the mixture starts to bubble.
In a small bowl, whisk together cornstarch and water until smooth.
Slowly add the cornstarch mixture to the saucepan, stirring constantly.
Continue cooking while stirring until the sauce thickens, about 1-2 minutes.
Remove from heat and let cool. The sauce can be stored in the refrigerator for up to a week.