An estimate is not a guess. It is the result of a rational procedure.

**Learning objective **Make models using imagination and three techniques:

- Make oversimplifying assumptions
- Break big problems into smaller problems
- Make estimates or even guesses to solve the smaller problems.

**Some tips**

- Ask: “What information would help me solve the problem?”
- Use rough data from readily available sources
- When in doubt, guess to nearest power of ten (logarithmic scale)

**Before class**

- Watch Powers of Ten
- Find out who Enrico Fermi was
- Read about the famous problem of the number of piano tuners in Chicago
- See how a Fermi problem can be tackled.
- Consult the main points of the lesson

**In class**

- Solve some Fermi problems, working in groups
- Orientation to project with Keeling’s Mauna Loa atmospheric CO2 data.
- Start R Chapter 3 on fitting models to data (pages 38-39, the utilities data).