Measuring the weight of a substance is easy. You simply place the object on a scale, and read the weight. Glitches can occur, however. If the scale is flawed, it will give a false reading. Because the density formula is dependent on weight, the scale needs to be highly accurate. Modern electronic scales are far more accurate than the "old style" mechanical scales, which were prone to drift out of calibration. Calibration refers to adjusting the scale to a highly accurate reference weight so that the scale gives an accurate reading with other weights.
Measuring volume is a little trickier than measuring weight. To make it easier, scientists developed a device called a graduated cylinder -- a clear tube with graduations marked on the side. The graduations are either metric or English measurements. Researchers prefer to use the metric system, since it's easier for other researchers in other countries to use. Suppose you have a rock, and you want to know its volume. You fill the graduated cylinder with water while reading the scale marked on the side. For instance, you could fill the cylinder with 10 cubic cm of water. Lower the rock into the water carefully so no water splashes. Allow the rock to settle on the bottom. Suppose you read the scale, and the water level rose to 14 cubic cm. Subtracting 10 from 14 leaves 4, telling you the rock is 4 cubic cm in volume.
The density formula uses both the weight and the volume of a substance. The formula for density is the mass divided by the volume. A difference exists between weight and mass in space or on other planets, but on the Earth, weight and mass are the same. In the rock example, suppose your rock weighed 6 g. The density is 6 g divided by 4 cubic cm. This yields 1.5. Therefore the density of the rock is 1.5 g/cubic cm.
Every element has its own unique density. For example, aluminum is a very light metal, and has a low density of 2.702 g/cubic cm. Lead is a very heavy metal, with a density of 11.35 g/cubic cm. Gold is even denser than lead at 19.32 g/cubic cm. Because each element has its own unique density, determining the density of an unknown element will help you determine what the element is.