Worth repeating: 90% of the cells of your body are not you. They are aliens that live there. They believe the sole purpose of your existence is to provide a home for them. Tomorrow will be my third day teaching Biology I so I know this to be true. I am substituting in Urbana - this week at Urbana High School. I can only substitute 120 days a school year (a Teacher's Retirement rule). My goal is to substitute at least 100 days. I am listed as being able to substitute any grade and any subject.