Dragonballs
The eternal dragon Shenron was originally a model made by Mr. Popo. Kami's assistants' passion was creating things such as models, and he had made a small structure of a Dragon, only about the size of a hand. He asked Kami if he could use his almost unlimited power to give life to his creation, and God agreed. Kami had created the Dragonballs to go along with this, and for the Dragon to be summoned, all seven must be collected. If Shenron is killed, then the model brakes, and you can't revive Shenron or use wishes until it is fixed. The plus side is that it can be mending easily and almost instantly. After Shenron's first death, Kami wasn't going to revive him, but after he saw how pure Goku's wish was, he decided on keeping the Dragon for people like him