One of the basic terms used in many natural sciences is density, a physical property defined as an object's mass divided by its volume. This means that to measure density, you typically need to measure an object's mass and volume separately, then calculate its density by dividing the mass by the volume. To measure mass and volume, you need the use of several basic laboratory tools.
Mass is one of the most easily obtained measurements. Use a scale or electronic balance to determine the object's weight, or mass. This measurement is usually represented in ounces or grams for English and metric systems respectively. When measuring the mass of a liquid, first weigh the container and then tare the scale before adding the liquid.
The most accurate way to determine an object's volume, especially in the case of an irregularly shaped object, is to immerse it in water and measure the amount of water it displaces. A graduated cylinder large enough to hold both the object and enough water to fully immerse it is the best tool for this job. A graduated cylinder can also tell you the volume of a liquid simply by pouring it into an empty cylinder. Don't use a beaker to determine volume, since the scale printed on the side of a beaker may be less accurate that that on a graduated cylinder, which is especially designed for measuring.
A hydrometer is a special device used to determine the density of a liquid. Rather than measuring the volume in a graduated cylinder and then needing to weigh the liquid to determine its mass (and, of course, subtracting the weight of its container), a hydrometer will float at a different level within a liquid based on its density. Different hydrometers use different scales for measuring density, so follow all instructions and measure carefully.