Here's why:

* Glycolysis is the first stage of cellular respiration. It breaks down glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon molecule.

* Energy Investment Phase: Glycolysis begins with an investment of 2 ATP molecules. These are used to phosphorylate glucose, making it more reactive.

* Energy Payoff Phase: The subsequent steps of glycolysis generate a total of 4 ATP molecules.

* Net Gain: Since you used 2 ATP and gained 4 ATP, the net gain of ATP in glycolysis is 2 ATP.

In summary, glycolysis produces 4 ATP but uses 2 ATP, resulting in a net gain of 2 ATP.