Basic concepts

On our backchannel discussion forum, we ScienceBloggers have been discussing a rather interesting idea that we could use to provide extra value to our readers. The idea is to do short articles about basic concepts in science. It’s an intriguing idea, and, while I could try my hand at posts about a number of basic concepts in science, I think that, given my background, my strongest contribution would be basic concepts in my areas of expertise. These could include questions such as:

What is cancer?

What is a randomized clinical trial?

How does chemotherapy work? (Certainly The Cheerful Oncologist could also answer this one and do it perhaps better than I.)

How does radiation therapy work?

Both Chad and Jake are asking their readers for suggestions, and Mark has already started by posting an explanation of means, medians, and modes, as well as normal distributions; so I thought I’d ask you: What sorts of basic concepts about science or medicine would you like me to try to explain in a brief, hopefully pithy post. At this point I’m just looking for ideas. I’m not sure how well I could do this, given the my tendency towards long-windedness, and I tend to like to pontificate about things more than explain basic concepts, but I’m game to give it a try and see how it goes.

All I need are examples of basic concepts that our readers are interested in having explained.