Jokes aside, you can microwave small smooth metal vessels. Like mixing bowls and such.

Do so at your own risk, if it doesn't specify that it's microwave safe.

But you can buy ones that specifically say they are microwave safe. It just means they're extra smooth, nowhere for charges to accumulate or concentrate. Might have something to do with it's size too, not being resonant at 2.4ghz, but don't quote me on that part.

Useful if you don't want to take leftovers out of the plastic container first, and if you've had glass explode on you before.