Can you microwave grapes?

A grape is roughly the wavelength of microwave energy inside watery tissue, so a pair of grapes touching acts as a resonant cavity that concentrates the field at the contact point. That ionises the sodium and potassium coming off the skin and produces a visible ball of plasma. It is a real effect, not a myth, and it is also a fire that is sitting inside your microwave. Plasma at the top of the cavity burns the waveguide cover and can ignite anything else in there.

What goes wrong

• Plasma burns the waveguide cover and the cavity paint
• The magnetron can be damaged by reflected energy with nothing to absorb it
• Scorched grape skin and smoke set off smoke alarms and can ignite

What to do instead

• Watch a recording of it instead of running it in your own oven
• If you want warm grapes for a recipe, roast them at 200 °C for 15 minutes

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 water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.