Department of Science Teaching, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel. Tel.: +972-8-9342273, Fax: +972-8-9342279, Machluf.yossy@weizmann.ac.il .|Department of Science Teaching, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel. Tel.: +972-8-9342273, Fax: +972-8-9342279, Machluf.yossy@weizmann.ac.il .