locking things

So - this morning Joseph comes into the living room looking so sad and forlorn.
J: "I can't go pee pee or poopy in the potty any more momma."
me: "why not?"
J: "I just can't."
??????

Fully understood that conversation a few minutes later when I tried to open the door to the powder room only to find it locked from the inside. Apparently some little fingers discovered that little lock on the back of the door.