SpongeBob wants to look inside, but Patrick says if he does so, it would ruin their friendship forever. SpongeBob thinks Patrick's box possibly contains stolen diamonds, severed heads, an Albino Jellyfish, or an embarrassing photo of him from the Christmas party.
SpongeBob becomes so desperate to see what is inside that he decides to break into Patrick's rock while he's sleeping and take a look. When SpongeBob looks inside the box, he realizes it only has a piece of string, but when really pulled, the string opens a secret compartment inside the box, revealing the actual item inside was indeed an embarrassing picture of SpongeBob at the Christmas Party.
"Lost and Found"
It can be seen stacked on the shelves of the Lost and Found among many other episodic objects.