motivation

Rewarding a task you enjoy kills your intrinsic motivation.

The foundational 1973 study by Lepper, Greene, and Nisbett demonstrated the effect using art and drawing activities, not puzzles.

The study involved drawing with felt-tip markers rather than playing with puzzles.

You turn a labor of love into a job the moment you demand a price for it.

Where this comes from

Study: Lepper, Greene, and Nisbett (1973)
What the check found: The core psychological phenomenon is accurately described, but the specific task used in the referenced 1973 study was drawing/art, not puzzles.
Confidence: Solid, with limits

One of these every day, and every one names the study it came from. When a finding is contested, it says so.