Tiring of fun
Work is what happens when you do something too long.
My mom one day asked me to clean some box fans, so I unscrewed some of the grates and wiped down the blades one by one. Right in the middle, my brother, Zach, asked what I was doing. I told him and asked if he’d like to help. Sure enough, he did, and began unscrewing the rest of the grates.
After a while, though, he tired of the fun and asked if he could stop. That’s when play became work, because he’d done it too long.