Using too high of heat is usually the reason for hard Rice Krispie squares. If the marshmallow and caramel get too hot, the sugar will start to change into more of a hard candy. For the best results, we recommend keeping your heat to low (even though it takes a bit longer). If you do use medium heat, watch very closely to make sure you aren't overcooking! We don't recommend medium-high heat as this is very likely to burn your Rice Krispies...and no one likes a burnt dessert recipe.