Not really a box, but its your testicles.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

When a man ejaculates into a woman’s vagina l, it becomes a splooge-box. His semen plus her vagina equals a splooge-box.