After SpongeBob tells Patrick some of his secrets to try and make him show him what is inside his secret box, the latter refuses, saying that while the former is an open book, he is more complicated, adding that "the inner machinations of [his] mind are an enigma." Patrick then imagines a live-action carton of milk spilling itself on a carpet.
Plankton tells a story to SpongeBob in hope of making him cry. Near the end of the story, Plankton spills a carton of milk that SpongeBob placed, making him cry when he finishes the story. This is a reference to the idiom "don't cry over spilled milk."