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). . If one power of ten seems too low and the next seems too high, try their geometric mean.

**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.
- Peruse Frontiers of Science Chapter 2 (actually the whole book is interesting)
- Explore screencast: Practical advice on solving Fermi problems, and some examples in the second half
- Consult the main points of the lesson

**In class**

- Solve some Fermi problems, working in groups

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