A prefabricated adaptable lightweight hollow core construction system comprising at least a unit (30) which may be extruded of plastic and which comprises a plurality of compression-resistant, flexibly connected, elongated, hollow element pairs (40). Each pair comprises a triangular base element protruding from one side of a flat flexible sheet and an octagonal head element flexibly connected to the uppermost angle of the base element. Element pairs have undersized spaces (42) between them adapted to receive elements in interlocking engagement. Adjacent head elements (36) may be manually swung apart on flexible connections (38) and forced into adjacent undersized negative spaces (42) to subsequently form rigid right-angle corners at incremental points (34a) or other shapes formed (41, 32a, 34c, 34d, 34e) along the unit. Elements of a second unit (30a) may be alternately and sequentially interlocked with those remaining elements of the first unit that are not engaged in corner formation. This rigidly extends the straight sides of right-angle corners. Forced interlocking produces high compression/tension values with concomitant rigidity within and throughout assembled constructions. Force to produce high compression/tension values is generated by leverage action of partially interlocked elements (36), one upon another between the manually driven element and the element with the highest values. Units may have an additional integral fastener (52) and/or auxiliary fasteners (54, 55, 56, 57, 58) to secure end elements (48), and rigid connectors (60, 66), or flexible connectors (61) for attachment of additional units and accessories (62, 64, 68); a cutting guide for accurate removal of unit portions (44). The unit requires no skill to assemble and can be reused. Unused portions may be used in future constructions and existing constructions may be disassembled and its units readapted into new constructions.