3 ideas
Harry is daydreaming in class when the teacher asks what has a long tail and revolves around the sun every 77 years. He answers 'a planet,' reasoning that all planets revolve around the sun, but the answer is a comet. Puzzling over the error, he realizes that 'All planets revolve around the sun' does not mean 'All things that revolve around the sun are planets,' and that a sentence which is true one way round can be false when reversed.
A sentence of the form 'All A are B' cannot be reversed while keeping it true, because A may be only a subset of B. When Harry proposes that no sentence can be reversed, his classmate Lisa offers 'No eagles are lions' as a counterexample, and the rule is refined: sentences of the form 'No A are B' do reverse validly. Each rule the children state becomes a hypothesis to test against new examples.
Rules of reasoning are not handed down by the teacher. Children build them together by proposing a rule, testing it against peers' examples and counterexamples, and revising it. Because knowledge comes out of shared dialogue about their own thinking, children practice philosophy rather than just receiving conclusions, and they learn that ordinary children can do this.