Can you microwave water?

Boiling needs nucleation sites — scratches, dust, dissolved gas — for bubbles to form on. In a smooth clean mug, straight out of the dishwasher, water can pass its boiling point with no bubbling at all. Then a jolt, a spoon, or a spoonful of instant coffee gives it somewhere to boil from and the whole cup flashes to steam at once. FDA has documented serious scald injuries to hands and faces from exactly this. Adding the coffee, sugar or tea bag before heating, and leaving a wooden stirrer in the cup, both prevent it.

How to do it properly

1. Put the coffee granules, sugar or tea bag in before heating, not after
2. Use a mug with a scratched or glazed interior rather than a pristine new one
3. Heat in short bursts and stop before the full time the mug looks like it needs
4. Let it stand for 30 seconds before reaching in, and never lean over the cup

What goes wrong

• Superheated water erupts on contact with a spoon and scalds hands and face
• A sealed bottle or jar of water builds pressure and can burst
• The mug handle can be hotter than the water suggests

Other "can you microwave" questions

• Can you microwave styrofoam? It depends — Only if the container is labeled microwave-safe, and most takeout foam is not.
• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave metal? No — No — metal reflects microwaves, arcs at edges, and can damage the oven or start a fire.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave cardboard? It depends — Plain dry cardboard is usually fine briefly; anything with glue, staples, wax or foil is not.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave ceramic? It depends — Yes for plain glazed ceramic; no if it has metallic trim or is unglazed and porous.