Science Puzzle
Why Does Food Get Hot in a Microwave but the Plate Doesn’t?
Heat a plate of leftovers in the microwave for two minutes. The food comes out steaming, but you can usually lift the plate by its rim without burning your fingers. In an ordinary oven, the plate would be too hot to touch.
The microwaves fill the whole oven and pass over the plate as well as the food. Why does the food get hot while the plate stays fairly cool?
The Answer
Microwaves mainly heat water, and food is full of water. A dry, glazed plate has almost none, so the microwaves pass straight through it.
A water molecule is slightly lopsided electrically: one end is a little positive and the other a little negative. Microwaves are a kind of light whose electric field flips direction billions of times a second. Each time it flips, the water molecules try to twist round to line up with it, jostling and bumping their neighbours. All that jostling is heat. Fats and sugars heat up in a similar way, though less strongly.
A plate made of glazed china or glass has very few molecules that can twist like this, so it barely absorbs the microwaves. It only warms up where it touches the hot food, which is why the middle of the plate gets warmer than the rim.
This also explains some microwave quirks. Frozen food heats unevenly, because the water molecules locked in ice can't twist easily, while any patches that melt heat up very fast; the defrost setting pauses to let the heat spread. And despite the popular idea, microwaves don't cook food from the inside out. They reach only a few centimetres in, and the middle of a large dish heats as warmth spreads inwards. According to the usual story, the microwave oven was invented after the American engineer Percy Spencer noticed, in the 1940s, that the radar equipment he was working on had melted a chocolate bar in his pocket.
The principle: Microwave heating. Microwaves make water molecules twist back and forth, and the jostling turns into heat, so moist food heats up while dry materials barely do.