No, it is not safe to put water-filled bottles or cups into a microwave oven for heating purposes. When water is heated in a closed container, it can become superheated, which means it can exceed its boiling point without actually boiling. This can cause the water to become unstable and erupt suddenly when disturbed or when a spoon or other object is placed in it. The rapid release of steam and hot water can result in serious burns.
Additionally, if the container is sealed tightly, it can potentially explode due to the pressure build-up from the steam inside.
To heat water in a microwave oven, it is recommended to use a microwave-safe container that allows steam to escape and to place a microwave-safe object, like a wooden stir stick or microwave-safe glass, in the container to prevent superheating. It's also advisable to follow the manufacturer's instructions and guidelines provided for your specific microwave oven model.