Elizabeth tells Jack that she wants to save Will, and Jack tells her about the alleged chest of Davy Jones, which would fulfill any desire. He gives her his compass and reveals that it doesn't point north: it points to the thing she wants most in this world.
Beakman builds a DIY compass using a needle and water, shows it in action and then explains the science with a short animation clip. Then there is a song about magnetism. Beakman then explains how the earth acts as a magnet and this is why compasses always point north.