Our glass jars are microwave and oven safe. To open a jar and break the vaccuum, hold the lid of the jar and pull the tab straight out, not up. To heat the food, remove the rubber tab, warm the food (lid on or off) directly in the glass jar either using a microwave or an oven. You can also transfer the food into a small pot and heat it on the stove top. Add in some water when heating meals to retain consistency and creaminess. Tip: You can also heat the meals in the morning and transfer them to a thermos to keep the food warm for a few hours so that your child can take them to school/nursery.