straws & string

 
Use monofilament fishing line pushed through straws to join them together. The monofilament line pushes through easier than ordinary string. Put the line through 3 straws. More or less loop the line into a circle and tie it. You have a triangle. Make shapes with a few sides such as a pyramid with four equilateral trianglar faces. Or make more complicated shapes with many sides. Cut straws shorter for smaller shapes.
 

 
The diagrams above show a tetrahedron, a cube and an octahedron. They are regular polyhedrons. Polyhedrons are three dimensional shapes made by joining flat pieces. Regular means the pieces are the same size.
 
Below are a twelve sided dodecahedron and a twenty sided icosahedron. All the line segments of a regular polyhedron are the same length. A sphere can be placed around a regular polyhedron so that the points where the line segments join are points of the sphere. There are many possible polyhedra but only five regular polyhedra.