Here's why:

* Glycolysis generates 4 ATP: During glycolysis, a glucose molecule is broken down, and 4 ATP are produced through substrate-level phosphorylation.

* Glycolysis consumes 2 ATP: However, two ATP molecules are required at the beginning of glycolysis to "activate" the glucose molecule.

Therefore, the net gain is 4 ATP - 2 ATP = 2 ATP.