Your MyDog class has an empty act() method, so put just one of the Dog objects in your world, and the rest must be objects of the class MyDog. If you do that, just one dog will move.

In your readme 'file', you say things like "up(); moves the Class up.". That's not true. up() isn't a class/static method, so it can't do anything with the class. You need to say "up(); moves the object up.", because you call the methods you created to objects, not to classes. By the way, you do this in gotomouse(): if (Greenfoot.mouseMoved(null) ) { MouseInfo mouse = Greenfoot.getMouseInfo(); setLocation (mouse.getX(), mouse.getY()); } It's better to do this: if (Greenfoot.mouseMoved(null) ) { MouseInfo mouse = Greenfoot.getMouseInfo(); if (mouse == null) return; setLocation (mouse.getX(), mouse.getY()); } Now, you return if the mouse info isn't available. When you do this, you have less chrashes. (little tip: change gotomouse() to goToMouse(), more readable ;) ) I hope this helps!