The following table shows the height of water (in cm) in a container after seconds of filling it at a steady rate:

A variable is a symbol, such as or , that represents values that can change.
If we represent the ordered pairs as points on a coordinate plane, the result looks like this:

A graph is a collection of points on the coordinate plane that represent the relationship between two variables and . By analyzing the graph, we can easily observe changes such as increases or decreases between the two variables.

For instance, the following graph shows the height of a specific cabin on a Ferris wheel from the ground over time. When is the time (in minutes) after the Ferris wheel starts, and is the height (in meters) from the ground, the graph shows the relationship between and :

As shown, patterns of increase and decrease that repeat in a consistent manner are called periodic changes.