The red particle has an electric charge, which you may set with the Charge slider bar. As long as it sits motionless, it feels no force. But if you click on the Fire! button, the particle will shoot to the right; and as soon as it moves, the magnetic field exerts a force upon it. The direction and magnitude of this force depend upon the cross product of its velocity and the magnetic field. We show you these vectors as little arrows attached to the particle: the blue one shows the magnetic field direction and strength, the red one the particle's current velocity, and the green arrow the magnetic force on the particle. Go ahead and click the Fire! button to see what happens.