If the mass of an item remains constant but its volume decreases, the density of the thing increases. If the volume of an item remains constant but its mass rises, the density of the thing increases. These changes in density are usually what is meant by saying that something has "dried out" or "watered down." Density can also be increased by mixing heavy objects with light ones or adding filler materials such as sand or gravel to make a lighter weight mixture.
Decreasing the density of some substances is important in physics for two reasons: first, because the less dense a substance is, the easier it is to lift; second, because the change in density associated with going from a liquid to a solid or a gas is always negative (except at absolute zero).
Increasing the density of a substance is important in physics for two reasons: first, because the more dense a substance is, the harder it is to lift; second, because the change in density associated with changing from a solid to a liquid or a gas is always positive (except at absolute zero).
These properties apply to any kind of matter, but they are especially important in physics because many physical processes involve matters of different densities interacting with each other.
When an object's volume is reduced but its mass stays constant, its density increases. This is because there is more space in the same amount of material. Denser objects are also heavier. A car's weight is its mass multiplied by gravity, so a lighter car is better.
If you decrease the volume of an object while keeping its mass the same, its density will remain the same as long as it's made out of the same material. For example, if you had a bag of sand and poured all of its contents into a glass, the density of the glass would be the same as the density of the sand (1 kg/L). Even though the glass has less volume than the bag, its mass is the same as the mass of the bag.
However, if you put the same quantity of glass into a second glass, then it would have a lower density because there's less space for the same amount of material. The glass industry knows how to make containers with low densities, for example bottles or jars. They do this by using materials with different densities. For example, they might use half glass and half plastic for its light weight.
The mass contained per unit volume is known as density. Because density is inversely related to volume, when an object's volume expands, its density falls while its mass remains constant. Thus, if an object's volume rises but its mass remains constant, its density drops. This is what happens when a gas fills up your car's tank: Its mass increases but its volume decreases, so its average density goes down.
Expanding objects tend to become less dense because their particles are closer together. As particles get closer together, they take up more space and thus reduce the object's volume. However, if an object's particles were to merge together into one big particle, its volume would increase even more than it is being reduced by having smaller particles, which would make it become more dense. So at least for now, feel free to think of expansion as a way for gases to reduce their average density.
One last thing to note about expansion: Gases always expand until there's no more room available near the surface of the object where the molecules began. Once all the spaces between molecules are filled with air, that's how much space they can give off. Any extra space inside the object is simply ignored by the gas since there's no more room available outside the object for it to go into again.