So... a scientific experiment. Measure the force it takes to move a stone in a bag on the two surfaces. Which would take more force?
The carpet took more force: 10 Newtons, compared to just 5 on the desk. There must be more friction on the carpet.
But Tibo said, "It's not fair. The bag makes it easier on the desk."
Half the class agreed. So we untied the bag, and tried the stone directly on the two surfaces:
Now the only question is, why didn't we agree that in the beginning?