Date: Feb 4, 2014 8:37 AM
Author: Michael de Villiers
Subject: Quadrilateral Inequality

Here's a simple quadrilateral exploration that is suitable for exploration by prospective or in-service mathematics teachers: 
http://dynamicmathematicslearning.com/trapezium-theorem-generalized.html
(A Java-enabled browser is required)

It generalizes a well known trapezium (trapezoid) result.