An actual put-down delivered by our 6 year old son, to our 9 year old son, in the car the other night:
Your face is stupider than a stupid sack of potatoes!
None of really knew what this meant, but we all laughed really hard.
This has “classic” written all over it, and is now destined to live on in our family as an inside joke, forever. Or at least until a better one comes along.
Feel free to use it without attribution. Our gift, to you.