1. Wait until Java finishes loading before clicking on anything. If you can't see buttons above, switch your browser to fullscreen view.
2. Initialize the demonstration problem by clicking on the "Initialize" button.
3. Click "Shoot New Bullet."  Click "Pause" to stop the bullet.  
4. The point of the problem is to hit the target.  Notice that the target has a constant speed of 25 m/s.  What must be the launch speed of the bullet in order to hit the target?  Enter that speed in the corresponding box.  You don't have to push Enter.  Now shoot the bullet.  (Note that you always have to click "Pause" to stop the bullet.  If you wish, you can use the step frame buttons to position the bullet precisely.)
5. Assuming you were successful, change the gun altitude to, say, 150 m.  How will this affect the launch speed?  Try it.
6. Change the gravitational field to that on the Moon, 1.6 N/kg.  How will this affect the launch speed needed to hit the target?
7. Initialize the problem again.  What must the launch angle be in order to hit the target?  Try it. (It's OK if the projectile goes off screen.  It should find its way back.)

If you were successful, you either knew the physics already or you learned something fundamental:  The horizontal motion of the projectile is independent of the vertical motion.  For a horizontal launch angle, the launch speed needed to hit the target was unaffected by gun altitude and gravitational field strength.  For a different launch angle, it was only necessary to change the magnitude of the launch velocity so that its horizontal component remained unchanged.