A couple of points of information, in order to understand how cute this is...
A tzadik (pictured right), in the Hebrew tradition, is a good and righteous person; a person who does not sin. It's also a letter in the alef-bet, the Hebrew alphabet. (Yes, I'm aware that the letter is actually tsade, but bear with me. I'm goy.)
Tof (Pictured left), pronounced "tough", is the last letter of the alef-bet and does not, as far as I know, have an extraneous meaning.
We've gotten The Boy interested in his alef-bet by making a big deal when he picks up the tzadik or tzadik sofit (the way the tzadik looks when it ends a word). We said, "The Boy is a tzadik!" He finds that funny, and so the tzade is his favorite letter in the alef-bet.
We have an alef-bet puzzle with which he plays. It's a foam puzzle that allows him to snap the pieces out, and he likes pushing them out and saying the names. Today, he pushed out the last letter and said, "Tof!"
I said, "Tof! The Boy is tough! Is The Boy a tough boy?"
"No, The Boy is a tzadik!"