The programme uses several examples to illustrate that: a result obtained by an intuitive method may not be correct. Based on established principles, an accurate conclusion can be deduced through the process of deductive reasoning. The programme introduces Euclid and his contribution to the study of geometry. What Euclid did was just like the idea of building a wall – he used definitions and axioms to build up the foundation layers, then on top of those and layer by layer, he developed various theorems to form an organized framework of geometry. The programme introduces the idea of a converse theorem of a geometrical theorem. It also uses an example to illustrate two thinking processes of formulating the proof of a geometrical problem: ? Forward deduction – to start with given conditions, use relevant axioms and theorems, deduce forward step by step to reach the required conclusion. ? Backward analysis – to start from the required conclusion, deduce backward the preceding steps to meet with the estashed facts.