The equal mass set, also known as the density rod set contains five bars of metal each with a mass of 30 grams, although they are different lengths. Included are tin, aluminum, steel, zinc, and copper. With this set, students use the water displacement method to find the volume of different rods that all have the same mass. They calculate the density of each rod and use the characteristic density of each material to identify all five rods. Then students consider the relationship between the mass, size, and arrangement of atoms to explain why different rods have different densities.