The second state of matter is the liquid. Liquids are an in–between state of matter. They can be found between the solid and gas states.

They don't have to be made up of the same molecules. If you have a variety of materials dissolved in a liquid, it is called a solution. One characteristic of a liquid is that it will fill up the shape of a container. The top part of a liquid will usually have a flat surface. That flat surface is the result of gravity pulling on the molecules.

Another trait of a liquid is that it is difficult to compress. When you compress something, you measure out a certain amount of material and force it into a smaller space. Solids are very difficult to compress and gases are very easy. Liquids are in the middle, but tend to be difficult. When you compress something, you force the atoms closer together. When the pressure goes up, substances are compressed. Liquids already have their atoms close together, so they are hard to compress. Many shock absorbers in cars, compress liquids in sealed tubes.

A special force keeps liquids together. Those intermolecular forces make sure that the molecules of the liquid stick to each other.