SpongeBob wanted to look inside, but Patrick said if he did, it would break their friendship forever. SpongeBob thought Patrick's box possibly contained stolen diamonds, severed heads, an albino jellyfish, or an embarrassing photo of him from the Christmas party. SpongeBob became so desperate to look inside he decided to steal it from Patrick's rock and look. SpongeBob saw the box only had a piece of string, but really when pulled, the string opened a secret compartment inside the box, revealing the actual item inside was indeed an embarrassing picture of SpongeBob at the Christmas Party.
The idea came to 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 was also mentioned in the song "Under My Rock," "I got a welcome mat, and dirty socks, a sink, a table, and a secret box."
It is in this episode, it can only be seen if you pull the string in the secret box releasing the secret compartment and the photo is inside.
Some people recall seeing in the box a picture of SpongeBob with a lampshade over his head or in his underwear while Patrick is revealing the embarrassing photo of SpongeBob at the Christmas party during the original airing of the episode and initial repeats.
When Patrick opens the secret box, he doesn't pull the string opening the secret compartment, he just looks inside the box and laughs.