Organizational/ Structural notes

### Exercises
- provide opportunity to practice what was just presented, and only what was just presented
- make it easy for students to focus on what was just presented, by depending on as little other skills and knowledge as possible
### Challenges
- go beyond the minimum understanding of what was just presented
- have students integrate prior skills and knowlege
#### Examples of sections with a good mix of exercises and challenges:
- [Numbers - Exercises and Challenges](http://nbviewer.ipython.org/urls/raw.github.com/ehmatthes/intro_programming/master/notebooks/var_string_num.ipynb#numbers)