Any science experiment will have three variables: constant, dependent, and independent. The independent variable is unique from the other two because it has the ability to stand apart from the others. In the experiment, it is the variable that you are changing or manipulating. That is why it is also sometimes referred to as the manipulated variable. When you change something about the independent variable, there will be a change in the dependent variable. That change is what you are measuring or observing in your experiment.
Let's look at an example. Suppose you want to measure how well different materials insulate a water bottle on a hot day. You have foam, wool, and cotton insulation, and you want to see which works best. The insulation material is what is being changed or manipulated. This is your independent variable. The temperature of the water in the bottle, which is what you are observing, is going to be dependent on that material.
As you can see from the example above, the independent variable can be identified as the part of the experiment that you manipulate or change. It is the thing that you are testing. Do not confuse this with the results of that change, which is the dependent variable.