The Theory of Special Relativity is such an area of physics that is almost impossible to demonstrate with experiments.  This demonstration shows the basic  theoretical experiment, and gives free hand in experimenting with a variety of different parameters.  It is possible to observe the events from different places, thus the approach of the Theory of Relativity may be easier to obtain.  

By setting the speeds to low values, you can see the classical physics to step in.  The menu on the left allow you to chose freely from the experiments.  These are based each other though, so if you are first-time-user, it is recommended to advance in proper order.

Have a nice time in Albert Einstein's real word!