In Exploration 5, you should have seen that speeding up the gas particles can help provide enough energy to the reactants so that rusting can occur. Before you sped up the gas particles, and had a high activation energy for the reaction, the reactants did not have enough energy to react. This is similar to what happens in reality. Oxygen and rust can be made to react with high energy when they are bombarded with ultra-violet (UV) light. Scientists believe that this is one reason why the surface of the planet Mars is covered in particles of rust. The visible surface of Mars that is brownish-orange is due to the vast amount of rust on the surface. 

In many types of chemical reactions, there is sometimes more than one way to get a chemical reaction to occur. Sometimes the reactants can be made to react more easily when they are in the presence of another molecule. In Exploration 6, you saw that the water helped the rusting reaction occur. This process also happens in reality, as the addition of water allows the atoms to move through a series of steps that require less energy, rather than going directly into making energy. This method of transforming reactants over a series of steps is referred to as a chemical pathway. The details of what happens in that pathway are not important right now. Instead, you should recognize that water decreases the amount of energy required to turn oxygen gas and iron into rust. 

Water is not used up in the chemical reaction for rust, it is simply a catalyst. A catalyst is a chemical substance that increases the rate of reaction without being consumed. The catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature.