Mathigon
Mathigon: Geometry: Polygons and Polyhedra: Platonic Solids
A Platonic solid is a polyhedron made up of only one kind of regular polygon, that looks the same from every direction. (In other words, the same number of faces meet at every vertex.) There are only five different Platonic solids:...
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Polyhedra Classification
This question and answer site provides a lot of basic information about polyhedra. The five regular polyhedra, "The Platonic Solids," are described.
Other
Platonic Realms: The Irrationality of the Square Root of Two
An informative resource on the nature of Platonic Solids and what constitutes these figures.
Other
Polyhedra: The Five Platonic Solids
A brief site that includes some activities to understand the figures.
Other
Jim Plank's Origami Page: Making a Truncated Tetrahedron
This website provides instructions on how to create a truncated tetrahedron.
National Council of Teachers of Mathematics
The Math Forum: Polyhedra Nets Paper
Contains six examples of polyhedra nets (cube, tetrahedron, octahedron, dodecahedron, icosahedron, cubosctahedron).
Math Is Fun
Math Is Fun: Making 3 D Solids
This tutorial features templates and instructions to make the following solids: tetrahedron, cube, octahedron, dodecahedron, and icosahedron.
Math Is Fun
Math Is Fun: Spinning Tetrahedron
Students learn about the terminology and formulas associated with a tetrahedron. The learning resource consists of definitions, a spinning tetrahedron, and examples.
University of Cambridge
University of Cambridge: Nrich: Skeleton Shapes
At this one page website a variety of polyhedra are shown. The challenge is to create the skeleton for each polyhedron and identify the needed materials. You can check your solution right at the website.
University of Cambridge
University of Cambridge: Nrich: Building With Solid Shapes
Predict which solid shape constructions would be unstable as shown at this one page website. Solutions are available right at the website.
ClassFlow
Class Flow: Nets
[Free Registration/Login Required] In this flipchart, students explore geometric sense by interacting with plane diagrams that create a three-dimensional shape.
Wolfram Research
Wolfram Alpha: Tetrahedron Vertices
The animation investigates the vertices of a tetrahedron.
Wolfram Research
Wolfram Alpha: Tetrahedron Edges
The animation explores edge truncation of a tetrahedron.
National Council of Teachers of Mathematics
The Math Forum: Angles of an Octahedron
This question and answer site from The Math Forum with Dr. Math discusses the angle between two adjacent faces of an octahedron. Links are also provided for additional information on related subjects.