The particles in a gas have the most energy of the 3 main states of matter. With so much energy, the particles in a gas are constantly moving at high speeds. The particles travel in straight lines until they hit a surface or another gas particle, which they will bounce off and continue straight in another direction. Due to their particle movement, gases have no real shape or volume. They will completely fill and take the shape of whatever closed container they are in. So each time you exhale, the particles from your breath spreads to fill the whole room! Gasses have a very low density due to the large amounts of space between their particles. The particles are so spread out, that most gasses are completely clear. This is why we usually do not notice the mixture of gasses all around us (air). When needed, gas particles can be squeezed closer together due to the space between them. For this reason gasses are compressible, though the more they are compressed the more pressure will build as the gas particle bounce off the walls of the container around them.
Gasses are described as having an indefinite shape and volume as each depends on the container they are in.
When placed into a container, a gas will expand to take the shape and volume of the entire container.
Examples of gasses include: steam, smog, oxygen, carbon dioxide, helium and air.
Gasses like helium are less dense than air, so our voice can travel through them faster than normal. This is why people sound so funny when they speak after inhaling helium. Gasses than are more dense than air will slow the voice down making, it sound low and deep. This is much more dangerous however, as dense gasses will sink into our lungs making them more difficult to breath out.
The States of Matter 