Density is a property of matter that is unique to each substance. It is a measure of the mass of the substance in a standard unit of volume.

Triple Beam Balance

Sometimes density is easy to sense. If two objects have exactly the same size and shape, the denser one may feel heavier. But if their densities are very close together, it will be hard to tell a difference. It gets really tough if you are dealing with materials that have very different sizes and/or very different shapes.

The only way to decide the density of a substance is to measure its mass and its volume, then divide.

If you want to measure the density of an object, read Skills: How is Density Measured?, it will tell you what to do.