There isn't a set number of "variables" in science. The number of variables depends on the experiment or study you're looking at. However, we can talk about the types of variables commonly used in scientific research:

1. Independent Variable:

- This is the variable that is manipulated or changed by the researcher.

- It's the "cause" in a cause-and-effect relationship.

- Example: In a study about the effect of fertilizer on plant growth, the independent variable would be the amount of fertilizer used.

2. Dependent Variable:

- This is the variable that is measured or observed in response to changes in the independent variable.

- It's the "effect" in a cause-and-effect relationship.

- Example: In the plant growth study, the dependent variable would be the plant height (which is expected to change based on the amount of fertilizer).

3. Controlled Variable:

- These are variables that are kept constant throughout the experiment to prevent them from influencing the results.

- They are important to ensure that any changes observed in the dependent variable are directly due to the independent variable.

- Example: In the plant growth study, controlled variables might include the amount of sunlight the plants receive, the type of soil they are planted in, and the amount of water they are given.

Important Note: You might sometimes hear about "extraneous variables" which are uncontrolled factors that could potentially affect the results. Scientists try to minimize the impact of extraneous variables by careful planning and design.