what’s the IV, DV, Constant, Control?

The independent variable is the variable that changes throughout the experiment. In this case, the IV is the amount of water. The height of the pea plant is dependent on the amount of water given. Not the other way around (The amount of water does not depend on how tall the plant is).

Therefore, we can conclude that the height of the pea plant must be the dependent variable. The dependent variable is what is being tested. It changes depending on when the independent variable changes.

There can be plenty of constants in an experiment, but it's important to name the ones that have the most impact in the experiment.

i.e. the color of the room is a constant if the plants are in the same room throughout the experiment, but it's not nearly as important as the type of soil being used to grow the plants in.

Some constants could be: pH of the water given, type of soil, type of pot that contains the plant, the pea plant clones, humidity, temperature, sunlight, etc.

A control is something that is used as a standard to compare the results of whatever is being experimented. In this case, the fourth plant that does not receive any water is the control. It is not affected by the independent variable, therefore, it can be used as the standard for a comparison later in the experiment.