Unusual Property
Identify four different nonzero numbers, a, b, c, and d, so that
a/b + c/d = (a+c) / (b+d)

Related External Resources 
No Matter What Shape Your Fractions are In
A geometric approach to fractions.
Visual Fractions [ java applet ]
A series of java programs using number lines to help visualize fractions and the operations on them.
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