Iron filings
Iron filings are very small pieces of iron that look like a dark gray powder. They are a common by-product of metalworking. Iron filing are often used in schools to show magnetic field lines. When filings are placed in a magnetic field, they become small magnets. This is because iron is a ferromagnetic material. The south pole of each particle attracts the north poles of its neighbors. In this way filings create chains parallel to the direction of the magnetic field.