Here's why:

* The purpose of experiments: Scientists conduct experiments to understand how one variable affects another. To isolate the effect of a single variable, they keep all other variables constant. This is called a controlled experiment.

* Independent and Dependent Variables: In a controlled experiment, the scientist manipulates the independent variable (the one they're testing) and observes the effect on the dependent variable (the one they are measuring).

* Examples: Imagine testing the effect of fertilizer on plant growth. You'd keep the amount of sunlight, water, and soil type constant, but vary the amount of fertilizer.

In summary: Scientists only keep variables constant that they are *not* trying to investigate. They do this to ensure the observed changes are due to the variable they're manipulating, not some other factor.