Triangles are strong because of their inherent structural characteristics. The corner angles of a triangle cannot change without an accompanying change in the length of the edge. Therefore, in order to change a triangle’s shape, an edge must collapse.
Edges can be made much stronger than connections. Triangles hold large loads without collapsing or having their structure altered. Triangles are the only polygons that have this characteristic. For instance, triangles have three connections, while squares have four. When forces are applied to a square, a square is more prone to lose its shape. Engineers often add a diagonal through the middle of a square, effectively turning it into two triangles and making it stronger. Triangles are the fundamental building blocks of many contemporary structures. Their strong, inflexible structure makes them perfect for contemporary designs. They are used in architecture and construction to create structures that must bear a certain amount of weight but still have material strength limits. For example, bridge and trusses are often constructed around triangles. Disney’s Epcot Center is another recognizable example of a structure in which triangles create stability in a 3D structure. Of all the twodimensional polygons, the triangle is considered the strongest because of its inherent structural qualities.
Each side is anchored to the other two so that there are no degrees of freedom.
http://wiki.answers.com/Q/Why_is_a_triangle_so_str...

Plywood is strong because of the raw materials used and the machinery involved to create plywood panels. Strengthening plywood starts with the manufacturing procedure of slicing wood
http://www.ehow.com/info_8480758_plywoodstrong.ht...

Ants are relatively strong because of the scaling law. If you leave morphological differences aside and assume similar morphology for both humans and ants, you will be as strong (
http://www.quora.com/Ants/Howareantssostrong

The Triangle Inequality for Real numbers: prove x + y < x + y
http://www.chacha.com/question/whatisthestrong...
