I am currently a high school teacher, and I teach algebra, geometry and algebra 2. I majored in physics in college and have always enjoyed physics. I am required to teach my students how to add and subtract vectors, graphically and algebraically. My textbook only teaches the students how to add vectors, so I have had to find resources on the web to show them how to subtract vectors by adding a negative vector. Your applet show people vector b as well as -b, so they see how to add vector a to -b by using the parallelogram principle. I just wanted to say thank you very much and know how much I appreciate all the hard work you have done. I can only imagine how hard it is to be a physics professor and still have time to program all these applets.