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 is sleeping and take a look. When SpongeBob looks inside the box, he realizes it only has a piece of string, but once he is out of sight, he does not know when really pulled, the string opens a secret compartment inside the box, revealing the actual item inside, that 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.
The idea came to Derek Drymon because he, too, had a secret box as a child. "He started telling us about it. We wanted to make fun of him and use it."
The secret box is also mentioned in the song "Under My Rock," with the lyric "I got a welcome mat, and dirty socks, a sink, a table, and a secret box."
The photo can only be seen if one pulls the string in the secret box releasing the secret compartment where the photo is found.
When Patrick opens the secret box, he does not pull the string opening the secret compartment; he just looks inside the box and laughs.
The function of the string is foreshadowed when SpongeBob is held by Patrick when he sleeps; Patrick pulls on SpongeBob's tongue while muttering about the secret box, hinting that the string has to be pulled to reveal the secret compartment.