Oooooh thanks psr - that's not so much a question as a request for a new page on the subject!
The poles receive less energy in the form of sunlight than does the equator, all points inbetween are somewhere in between according to their latitude, so even on a perfectly stable and "all things equal" planet, the poles would be the coldest places.
Antarctica is colder again as it is:
- A landmass surrounded by ocean, most of the continent is unaffected by the effect of the surrounding seas that make the climate of coastal regions milder than inland regions
- The highest continent, temperature drops with altitude, so a "high" Antarctica is colder than it would be if it was low and flat instead of high and mountainous.
- The source of it's own weather systems that have very little to do with the rest of the planet for much of the year. In the Arctic for instance, the weather systems spill in or out of the Northern Hemisphere so making them less extreme. In Antarctica, the weather systems go round and round getting more and more extreme and not being diluted.
Hope this satisfies your friend for the time being!