Links learning for kids had a wonderful introduction on symmetry. It is clear and concise. Students can listen, see and interact with this lesson. It gives a wide range of symmetry. Students are able to look beyond the geometrical lines for symmetry. The lesson brings nature and surroundings to life in symmetry. Art also plays a big part in this lesson on symmetry. Though this lesson is for kids, I believe that the elementary way in which it was presented would be excellent for differentiation in a secondary lesson.
http://www.linkslearning.org/Kids/1_Math/2_Illustrated_Lessons/4_Line_Symmetry/index.html
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