\r\n" );
document.write( "I'll try to explain by looking at non-rigid figures.\r\n" );
document.write( "\r\n" );
document.write( "How to tell if a polygon is RIGID:\r\n" );
document.write( "\r\n" );
document.write( "If the polygon is made of wooden bars with a hinge at each vertex, none of\r\n" );
document.write( "the hinges can be turned, IF it is RIGID.\r\n" );
document.write( "\r\n" );
document.write( "Put a hinge at each corner of a rectangle and you can easily make the\r\n" );
document.write( "rectangle into a parallelogram. So a rectangle is NOT RIGID.\r\n" );
document.write( "\r\n" );
document.write( "
\r\n" );
document.write( "\r\n" );
document.write( "Suppose the little circles at the corners are hinges.\r\n" );
document.write( "Then you could easily transform the rectangle on the left\r\n" );
document.write( "into the parallelogram on the right, just by pushing on\r\n" );
document.write( "the left side, or by pulling on the right side. So a \r\n" );
document.write( "rectangle is NOT RIGID. Neither is a parallelogram. \r\n" );
document.write( "That's because you can make either change shapes\r\n" );
document.write( "just by pushing or pulling on a side.\r\n" );
document.write( "\r\n" );
document.write( "Now look at this triangle with hinges on the corners.\r\n" );
document.write( "
\r\n" );
document.write( "\r\n" );
document.write( "Can you change its shape by pushing or pulling on a side? \r\n" );
document.write( "NO! You can't make the hinges turn at all! (without bending \r\n" );
document.write( "the straight sides). So a triangle is rigid. \r\n" );
document.write( "\r\n" );
document.write( "In fact a triangle is the ONLY polygon that is RIGID.\r\n" );
document.write( "\r\n" );
document.write( "Edwin
\n" );
document.write( "![\"\"](\"/images/stars/stars-13.gif\")
