A vampire walks into a bar. The bartender asks if he’d like some blood. “Just hot water, please,” the vampire responds. The bartender raise an eyebrow, but goes to get the hot water. When he places the mug in front of the vampire, he says, “I thought you were a vampire. Why order hot water?” The vampire pulls a tampon out of his bag and says, “I’m having tea.”
You’re welcome.
If i recall, the main trick is to make sure there aren’t any sharp corners or other places that are close enough together that arcing can occur. But there might be some additional stuff, like coatings and other design choices that are necessary to promote even and effective heating (otherwise the microwave will heat the bowl instead of the food since the radiation won’t pass through metal like it does with plastic, paper, or glass).