In this lecture for Classes 3–5, students are introduced to triangles, their common properties, and their classification based on sides (equilateral, isosceles, scalene) and angles (acute, right, obtuse). The lesson explains key rules like the sum of angles (180°), the triangle inequality, and the exterior angle property. Students learn how different triangle types overlap or relate, such as why every equilateral triangle is also isosceles and acute, but not scalene, right, or obtuse. The lecture also connects properties with real examples, highlights symmetry, and explains special cases like right-angled isosceles triangles and the Pythagoras Theorem.