One of the simplest examples of mathematics used in understanding chemical reactions is based on two chemicals, A and B (they can be molecules or ions). If A and B encounter one another, they can rearrange themselves into molecules or ions of two other substances: in this example, C and D. The reaction that takes place can give off or absorb energy, making the molecules move faster or slower. Although this is a simple example of what can happen in a chemical reaction, it can still be analyzed using mathematical modeling. For example, given starting amounts of A, B, C, and D molecules at time *t* = 0, what would the molecules be like at time *t*_{1} (or after a specific amount of time)? These and more complex chemical engineering questions can be answered using mathematical modeling.