Two magnetic fields in the same space combine into one resultant field. A bar magnet's field collides with the Earth's magnetic field, and in some spots the two fields add together. In others they cancel completely, creating dead zones where a compass needle spins freely because no net force acts on it. You reveal this by placing a bar magnet on butcher paper and tracing the compass direction at dozens of locations around it. Connecting those points maps the curved field lines — and marks the null points where the two fields are equal and opposite.

The invisible force around a magnet spreads out in curved lines. You can see those lines in three dimensions by filling a plastic bottle of baby oil with tiny steel fibers cut from extra-fine steel wool. Hold a magnet against the outside and shake gently. The fibers swing into alignment and trace the curved field lines through the oil. Move the magnet around the bottle and the pattern shifts in real time.

The invisible force around a magnet can reach through cardboard and air to attract metal clips. Some materials, though, stop that force. You build a cardboard sandwich with a magnet on top and paper clips hanging from the bottom. Slide a popsicle stick into the gap and the clips stay. Slide in a metal strap and the clips fall. Materials that let field lines pass through are called nonpermeable. Those that absorb them are permeable — and only magnetic materials act as shields.

The invisible force around a magnet can weaken when the magnet heats up. You test five magnets at temperatures from 0°C to 100°C, pressing each one into a tray of steel washers and counting how many stick. At 0°C the magnets grab around 30 washers. By 100°C they pick up fewer than 10 — a dramatic drop across that range.