Here's why:

* The Goal: The Calvin cycle's primary goal is to fix carbon dioxide (CO2) from the atmosphere and use it to build sugars.

* The Process: The cycle starts with the carbon fixation of CO2 by the enzyme rubisco. This generates a 6-carbon molecule that quickly splits into two molecules of 3-carbon compound called 3-phosphoglycerate. This compound is then reduced to G3P.

* G3P's Importance: G3P is a 3-carbon sugar that represents the reduced form of CO2. It is a key building block for other organic molecules, including glucose, which is the primary form of sugar produced by plants for energy and growth.

To summarize: While the Calvin cycle indirectly produces glucose, the direct output of the cycle is glyceraldehyde 3-phosphate (G3P).