A construction kit including a plurality of construction assembly pieces each comprising a resilient half rod. The half-rod comprises a rod divided in half longitudinally and having a set of teeth. The set of teeth comprises teeth disposed at intervals along a longitudinal length of said half-rod located in alternate planes to either side of a longitudinal mid-plane of the rod. The teeth are configured to provide a snap-fit interlock with a corresponding set of teeth on the half-rod of an edge of a second corresponding construction assembly piece to define a full, preferably circular cross-section, rod where the two assembly pieces interlock. Each construction assembly pieces defines a flexible plastic rectangle partly open along one edge to allow the rectangle to be flexed further open, for modeling structures having fluid shapes.