OK, it's like this
When you heat something in a conventional oven, heat transfers in three ways:

1. Radiation - the infra red radiation from the hot element/gas flame impinges on the surface of the food and transfers energy to it, thus heating the surface. As food (a large amount of which is water) absorbs thermal radiation very well, it's said to have a high absorption coefficient, and so the IR radiation doesn't penetrate very far into the food. Then we have...

2. Conduction - Hot air in the oven also heats the surface of the food by a simple energy exchange. This heat, together with the radiant heating, transfers further into the food by thermal conduction, thus heating it through. Conduction through water and so on is not terribly quick. This is why the outside of your frozen chicken can be burned while the inside is still rock hard and at -10°C. It takes time for the temperature of the inside to reach the temperature of the outside. This is called thermal equilibrium. Actually, if you let your dinner reach thermal equilibrium it would be burnt, but that's just an aside. All you need is for the middle to be hot enough so that it's cooked.

3. Convection - This is a third heat transfer method which occurs in fluids. Non-uniform heating causes some parts of the fluid to become hotter than others. They expand, and become less dense and so travel upwards in the fluid as cooler denser fluid flows in under gravity, displacing the hot zones and causing mixing and heat transfer within the fluid. This is important in many areas, not just cooking, and is the primary driving force for the formation of thunderclouds, for example.

So now microwaves.

Well, microwaves heat by essentially the same methods. The wavelength is much longer than IR, but to all intents and purposes, the water in the food still absorbs the incident radiation. However, the absorption coefficient is lower, which means the microwaves penetrate further in to the food, and so the temperature difference between the outside and inside is much less pronounced. Because of the way microwaves are absorbed, the air inside the oven and the oven itself do not heat up appreciably, so crucially, the outside of the food can actually be cooler than the inside because it loses heat by conduction to the cooler air within the oven. This is the origin of the inside out heating myth.

Convection also plays a part. Because of the longer wavelength of microwaves (of the order of centimetres) standing waves can cause uneven heating of a fluid. So it is possible that part of a fluid can be hotter than other parts. This is why you should always stir your coffee before drinking it, if you've heated it up in the microwave.

Given enough time, convection will equilibrate the temperature, but in the case of yoghurt, it's so viscous that convection is very slow.

And that is why fearsome mumbler's mate ended up with a scalded cock! So the story may well be true, it's just the physics that's suspect.
( K2k6 has a proper job these days, Tue 11 Aug 2009, 9:28, 2 replies)

Cheers...
...that was really interesting.

And I actually mean that sincerely in case you thought my comment was slathered in invisible sarcasm.
(Rotimer was micturating windward at, Tue 11 Aug 2009, 16:53, closed)

Thanks...
K2k6 for explaining how the magic warming up box really works :)
(fearsome mumbler is shit at telling stories, Tue 11 Aug 2009, 23:00, closed)