Looking for a Gas
Gases are everywhere. There is something named the atmosphere. That's a big layer of gas that surrounds the Earth, extending from the ground to about 300 miles (480 km) into the air. In solids, atoms and molecules are compact and close together. Particles of liquids are generally a little more spread out. However, gases have no definite volume; they expand as much as they can. Their atoms and molecules are full of energy, bouncing around constantly.
Gases can fill a container of any size or shape. That is one of their physical characteristics. Think about a balloon. No matter what shape you make the balloon, it will be evenly filled with the gas's particles. The atoms and molecules are spread equally throughout the entire balloon. Liquids can only fill the bottom of the container, while gases can fill it entirely.
Compared to liquids and solids, gases can also be compressed with relatively little pressure. It happens all the time. Combinations of pressure and decreasing temperature force gases into tubes that we use every day. You might see compressed air in a spray bottle or feel the carbon dioxide rush out of a can of soda. Those are both examples of gas forced into a space smaller than it would want, and the gas escapes the first chance it gets.
You might hear the term vapor. Vapor and gas mean the same thing. The word vapor is used to describe substances that are normally liquids at room temperature, but are currently in their gas form. Compounds like carbon dioxide that are gases at room temperature don't need the term, so scientists will rarely talk about carbon dioxide vapor. Water and mercury are liquids at room temperature, so they're called water vapor and mercury vapor after they're changed to a gas.