Please don't use these slides to teach. Really. I only posted this as an example of how I used to (poorly) use PowerPoint. See the updated version here: http://www.slideshare.net/benwildeboer/journey-to-the-center-of-the-earth-265038
And see the explanation here: http://blog.benwildeboer.com/2008/as-promised-presentations-before-and-after/
Clipping is a handy way to collect important slides you want to go back to later.