Explain it simply

Feynman’s test: if you cannot explain it to a bright twelve-year-old, you do not understand it yet. The useful part is not the finished explanation. It is the moment you reach for a word that sounds clever and realise you are covering a hole.

Each note below says what was hard to explain and what to read about it. Every one has to pass its own jargon check before it can be published: the build fails if a note reads above the reading age it claims to be written for.

• Averages that lie

  An average is a fair summary when things cluster around a middle, and close to useless when a few cases are enormous.

  Reading age 5.4 · 2 gaps found

• Updating a belief on new evidence

  A test result does not tell you what is true. It tells you how much to move the belief you already held.

  Reading age 2.6 · 2 gaps found

• Compound growth

  Growth that feeds on itself starts slower than you expect and ends up somewhere you did not expect at all.

  Reading age 2.9 · 2 gaps found

• Feedback loops

  When a thing's output comes back round and changes its input, the system stops adding up and starts behaving.

  Reading age 3.7 · 2 gaps found

• Natural selection

  If things make copies of themselves, the copies differ a little, and some differences help, then the helpful ones take over. Nobody has to plan it.

  Reading age 4.1 · 2 gaps found

• Why queues blow up

  A queue that is nearly full does not get a little slower. It gets much slower, and the last few percent of use cost the most.

  Reading age 3.7 · 2 gaps found