Most of us know from experience that some things heat better in microwave ovens than others do. For example, if we put a cup of water in the microwave, the cup itself doesn't get particularly hot but the water inside it quickly heats to boiling point.
What do microwave ovens heat?
Some substances that do heat up in a microwave include: paper, nylon, wood, cheese, water and cooking oil. Examples of materials that are not significantly heated by microwave ovens include: polyethylene, glass and china. But rather than just taking our word for it, try this experiment to see which household food items are heated by microwave ovens and which are not.
Why do microwave ovens heat some materials and not others?
To understand why different materials behave differently in microwave ovens we first need to know what goes on inside them. Every microwave oven has a device called a magnetron hidden away inside it.
This device is a bit like a radio transmitter - it generates the microwaves, which it sends out into the main cavity of the microwave oven. If you look inside your microwave oven you will see that there is a plastic panel about 10cm/4in square in one side. This is where the microwaves enter the main oven.
What are microwaves?
But what are these microwaves? Most people are familiar with the idea of light being made up from waves (we call them electromagnetic waves or radiation). The light we can see is made up of waves of different 'wavelengths' (the distance between successive peaks and troughs in the waves). Blue light has a wavelength of about 0.0004mm and red light has a longer wavelength (around 0.0007mm).
Electromagnetic radiation can have any wavelength. If the wavelength is very much smaller (around 0.000001mm) we call them X-rays; if is much longer, say around 100m/110 yards, we call them radio waves. Microwaves are just electromagnetic radiation with wavelengths of around 10cm/4in.
Why this particular wavelength? Just as some substances absorb particular wavelengths of light, giving them their colour, so it happens that water molecules absorb electromagnetic radiation in the microwave region. The energy the molecules gain makes them move about rapidly and as they bump into neighbouring molecules they get 'excited' as well and the water gets hotter as a consequence.
Why are microwaves so good at heating water?
 Water consists of two hydrogen atoms joined to a single oxygen atom. In practice the oxygen atom is usually more strongly bound to one of the two hydrogen atoms at any particular time, so we tend to think of water as being from a 'hydroxyl' group (the oxygen with one hydrogen joined to it) bound with a hydrogen atom.
When a water molecule absorbs a microwave, the hydroxyl group gets excited and starts to rotate. Many other molecules contain hydroxyl groups and most of these will also absorb microwaves and so become hot in the microwave oven. If you try the experiments above you'll find that the substances that get warmer generally have a lot of hydroxyl groups while those that do not have few, if any.
Microwaves don't heat ice!
For microwaves to be able to heat substances that contain hydroxyl groups, those hydroxyl groups need to be able to move freely. They can do this in liquid water, but not in solid water (ice), so microwaves tend not to be absorbed much by ice.
If you make a beaker out of ice (see experiment below) you can use it in the microwave to heat water. The ice only melts when it is in contact with the hot water - it is hardly heated at all by the microwaves themselves. You can try this easily for yourself with a simple experiment to make tea in an ice beaker.
How do microwave ovens defrost frozen food?
This might make you wonder how microwave ovens defrost frozen foods. The answer is that when you freeze water that has sugar or salt (or anything else) dissolved in it, its freezing point is reduced (this is why sea water freezes at around -4C/25F and not at 0C/32F).
 The higher the concentration of salt or sugar, the lower the freezing point (up to a limit that will not be reached in a domestic freezer). Importantly, though, as the water starts to freeze, only the actual water freezes, leaving the salt or sugar behind in the solution, so the concentration of the solution increases and the freezing point falls further. Thus (at least in domestic freezers) frozen food still contains some liquid water.
It is this liquid water that the microwave heats. The hot water then melts some of the ice surrounding it, making more liquid for the microwaves to heat and so on, until the food is fully defrosted.
Why is food cooked in a microwave oven sometimes not cooked uniformly?
Inside the microwave oven, the microwaves bounce off the metal internal walls and set up complex 'standing wave' patterns. As with any wave, microwaves have peaks and troughs and the intensity of the microwaves is greatest in the peaks and troughs and lowest at points in between.
So if some food is near one of the peaks it will absorb lots of microwaves and get really hot, while if it is midway between peaks and troughs it may receive hardly any microwaves and so not get very hot at all.
You can measure the distance between the peaks and troughs with a simple experiment - and even use the result to calculate the speed of light!
What's the purpose of a turntable in a microwave oven?
One way that microwave manufacturers use to overcome the problem of non-uniform heating in microwaves is to put turntables in the ovens so that the food keeps moving around and has a good chance of passing through plenty of the hot-spots.
Of course this is never completely perfect - no matter how well designed the microwave is food simply will not cook uniformly because the amount of water (and hydroxyl groups) in real food is not always uniformly distributed. Even with the best turntables no two parts of the food will experience exactly the same amount of microwave intensity as they move around. This is a very simple experiment using a potato to see how uniformly your own microwave cooks:
Why are we told not to put metal in a microwave oven?
The reason is simple. Microwaves are electromagnetic radiation. When such radiation reaches metals (or any electrical conductor) it induces an electric current in the metal - this is how radios, televisions and mobile phones receive their signals. However, there is so much microwave power inside the microwave oven (typically around 800 watts) that if it is all caught by an 'aerial' (which is exactly what any pieces of metal in the microwave will become) it can induce rather high currents.
High currents lead to high voltages and those, in turn, cause sparks to fly around inside the microwave from the metal objects earthing to the microwave case, which could damage the microwave. Flying sparks can also be alarming, so it is best to follow the manufacturer's instructions and avoid putting metals in the microwave oven.
Why does food cooked in a microwave sometimes lack flavour and colour?
 We know that microwaves heat water. As there's so much water in our food, all that happens is that the water gets heated up to its boiling point (around 100C/212F) and the food never gets any hotter (at least not until all the water has boiled away and it has dried out and become inedible!) .
This is why food cooked in a microwave often appears to have little flavour and never gets any brown colour. The chemical reactions that cause browning are only able to operate at high temperatures (well above 100C/212F), so they won't happen in a microwave oven, unless we do something to get the temperature higher.
Creating colour
You can coat microwave cookware with a layer that absorbs the microwaves very strongly but that doesn't rely on water to do so. As a result, its surface can be heated to higher temperatures and thus heat the food as if it were in a frying pan on the stove.
In practice the easiest way to get a high surface temperature on such a browning dish is to coat it with a (poorly) conducting layer, so that electric currents are induced. The browning dish acts like a mini electric hotplate, heating the food locally to a temperature well above 100C/212F, which causes the food to brown.
Do microwaves heat from the inside out, rather than the outside in?
There are many myths about how microwave ovens operate. One is that microwaves somehow heat from the inside out. This is completely false. Microwaves are absorbed from the outside, so they heat from the outside inwards.
 If you heat a jam doughnut in the microwave the jam can get so hot it will scald your mouth if you eat it, while the dough remains cool enough to eat
However, the microwaves heat water (or other hydroxyl containing molecules), so if the outer regions of a particular food contain less water than the inner regions, the middle may end up hotter than the outside. One example of this is the jam doughnut. The dough of the doughnut contains lots of air bubbles so overall it has much less water than the jam in the middle. If you heat a jam doughnut in the microwave the jam can get so hot it will scald your mouth if you eat it, while the dough remains cool enough to eat.
Discuss the results of your experiments, and any food combinations you've tried, with fellow foodies on The Food message board.
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