Necklace: Here we observe the sequence of the child’s necklace ( Sec. 608 ). The child starts with an enlargement of his mother’s necklace consisting of a dozen half- inch-by- 12-inch aluminum tubes strung tightly together on dacron cords. The child drapes the necklace over his shoulders to assume various shapes. Then, removing one tube at a time, he finds that the necklace remains flexible … until all but the last three are removed and__as a triangle__it suddenly holds its shape. (Thus we arrive at the triangular definition of a structure.)