Eggs. Still in their shells.
Fun to watch but messy to clean up after.
Whole eggs and kitchen foil – 2 things you should never put in a microwave.
Someone once showed me how to boil an egg in a microwave: wrap it in foil, then submerge in a pot of water and zap it.
I thought he was having me on, but apparently because the foil is underwater it doesn’t react badly and because the egg is wrapped in foil it doesn’t get heated directly and so doesn’t explode. Clever, but scary looking!