This web page provides information and a graphical exercise for students regarding the interaction between magnetic field lines and a plasma. The activity involves tracing a typical interplanetary magnetic field line, dragged out of a location on the Sun by the radial flow of the solar wind. This illustrates the way magnetic field lines are “”frozen to the plasma”” and the wrapping of field lines due to the rotation of the sun. This is part of the work “”The Exploration of the Earth’s Magnetosphere””. A Spanish translation is available.