Grainy caramel sauce happens when the sugar crystallizes. This can be a result of undissolved sugar crystals, temperature fluctuations, or a host of other factors. But don't worry if your creamy caramel sauce becomes gritty. It's so easy to fix!  Place the caramel sauce and a tablespoon or two of water in a saucepan over medium heat. Whisk and bring the mixture to a boil until it's smooth again. You can also reheat it until bubbly in the microwave. Cool the caramel completely at room temperature before storing it in the refrigerator.