Liquid Basics
The second state of matter we will discuss is a liquid. Solids are things you can hold that maintain their shape. Gases are floating around you or trapped in bubbles. What is a liquid? Water is a liquid. Your blood is a 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. If you pour some water (H2O) in a cup, it will fill up the bottom of the cup first and then fill the rest. The water will also take the shape of the cup. The top part of a liquid will usually have a flat surface. That flat surface is the result of gravity pulling on the molecules. Putting an ice cube (solid) into a cup will leave you with a cube in the middle of the cup because it is a solid. The shape of the solid cube won't change until the ice becomes a liquid.
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. 