Problem with smooth shading

I am completely new to bender, using the noob to pro tutorial on wikibooks to get started, as the title suggests, things are happening that aren’t outlined in the tutorial. I’m working with Blender ver. 2.57

In the tutorial, one is tasked to build a rudimentary model of a person, the head is an icosphere mesh. The next step is to apply the “subdivision surface” modifier and set the shading to smooth. However, a pattern of strange dents appear on the head. http://imgur.com/wihh9.jpg

Any thoughts will be appreciated.

Some of the faces have their normals facing the opposite way to the others. Select all the vertices (A) and recalculate the normal direction (Ctrl+N)

oh, wow, thanks!

For interest’s sake; why does this happen?