Place a heavy heat and water-safe item on the jar's top to weigh it down; a clean rock works well. Also, an occasional mishap can result in a broken or cracked jar - it happens to the best of us without rhyme or reason. To help prevent this, use a mason jar specifically meant for canning or cooking. Do not use leftover glass jars from other products. Also, use a brand-new lid so you know the seal is good and the contents inside will stay dry.