(- )–

-

-

http://www.youtube.com/watch?v=qGrWqEEDxY0

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.1

.2

.3

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http://www.youtube.com/watch?v=BOMxHNYw0jg

http://www.youtube.com/watch?v=fWNKQ9jGmiM

http://www.youtube.com/watch?v=eOvWioz4PoQ

http://www.youtube.com/watch?v=DEoT_EH_Uss

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http://www.youtube.com/watch?v=b3NXsgjPSQo

http://ofek.cet.ac.il/units/he/science/unit93/act1.aspx?nUnit=93&sSubjectKey=science