We introduce Easy Java Simulations, Ejs, a tool created by science teachers to help teach and learn science. Ejs allows users to create simulations using their knowledge of the scientific model. The author needs to supply a small amount of code for the model; the tool provides a graphical drag-and-drop interface to build the program. The resulting Ejs generated program is an independent, high quality Java application or applet ready to be published on a Web server. Ejs can serve as an effective teaching and learning tool if used in an appropriate pedagogical setting; for instance, to help students create their own simulations in order to express their conceptions on how a given scientific process works.