There are lots of different types of gas. Gases have different properties, just as solids and liquids do, and we use them for different purposes.
Because many gases are colourless and odourless (they have no smell), we need to be careful when using them. Some useful gases are also poisonous.
The natural gas we use to heat our homes and cook with has an odour added to it before it is piped into our homes so that we can detect leaks. We use natural gas because it burns easily to provide us with energy, but it is also explosive.
Without oxygen life on earth would be very different! Organisms use oxygen to release energy from food in the process called respiration. Green plants make their own food using light to combine carbon dioxide with water to make sugars. This is called photosynthesis.