Thank you very much for your responding. By the way, I want to ask you wherether I can modify a lite bit something in the code of your simulation (absolutely I still show your source as well as your web page)? The reason for that is I need to translate the language in the instruction from English to my language so that my students can understand. Some of simulations that I downloaded did not have java files. Can I ask you for the code? Many thanks Benson