Ben - Now that's a good question. I think he's probably referring to the idea that if you stop washing your hair for six weeks or so, then it ends up sort of auto-cleaning and you end up with clean hair despite it not being washed. I think the truth is a little bit more complicated than that. It's not that your hair is clean; it's that your hair has the right amount of oils. Normally, when you wash your hair with shampoo, it's a detergent and it will strip the oils off your hair. Hair is supposed to be a bit oily so your hair follicles produce additional oil to make up for the oil that's been washed away and you end up getting greasy hair. If you don't wash it away, there seems to be some kind of feedback mechanism that means that the follicles will produce the right amount of oil, and you won't end up with greasy hair.

Now I don't know if this really bears itself out in practice and from what I have seen and read about it, it seems to be much better for people with very short hair. It's not that they never wash their hair, it's just that they don't use soaps or shampoos - they just wash it with water. The water takes away the dirt and grime but the oils stay there to help protect the hair. Dave, you've got quite a lot of hair. How easy is it for you to manage?

Dave - A while ago, purely in the spirit of scientific inquiry, and much to the annoyance of my then girlfriend, I decided to see what would happen if I didn't wash my hair for three months. Basically, what happened is my hair sort of reached an equilibrium. It was much less greasy than it would be just before you'd wash your hair normally, but still greasier than just after it: a bit greasy, but not very greasy. So there does seem to be some kind of feedback loop. Also, interesting things happen when it rained because it appears that your natural grease from your hair is quite good at waterproofing because the outer layer got incredibly greasy and the rain would run off whilst the hair underneath would be entirely dry. So obviously, natural oils have advantages over conditioners.

Ben - That's very strange. It's like wearing a natural shower cap made only of hair - that's revolting.