You can build a bridge of loose iron nails between two magnets, which are embedded in a table top and which are opposite each other in a not too big distance with south pole and north pole. You have to use a lot of nails to build a stable bridge arch.
Further informationson the exhibit "Magnetic bridge"
Magnets generate a magnetic field in their environment. With two magnets, which lie opposite each other, a field is created which - similar to a dome above a hall - curves from one magnet to the other. The distance between the south pole and the north pole of the magnets should not be too great. To illustrate a magnetic field, so-called field lines are used. These lines face at each point in the direction of the force that the magnetic field exerts on magnetic materials. The distance between adjacent field lines is a measure of the strength of the field. The smaller the distance, the stronger the magnetic field. In our experiment, the two magnets magnetize the iron nails, which thus become small magnets with north and south poles. These then arrange themselves along the field lines and hang - laterally and magnetically connected in length - on each other. In this way, a bridge can be created from magnet to magnet, which holds together all by itself.