Since it isn't yet noded, a jack-in-the-box is an old children's toy whose very existence shows that the human race has an evil, sadistic streak that needs to be checked on the regular.
What it is is a combination of music box and evil surprise. A hand crank on the side of an otherwise colorful yet unassuming box does two things simultaneously - first, it rotates a metal cylinder festooned with bumps past different lengths of fingers of metal which when displaced by the bumps play musical notes. The most famous song of which is "Pop goes the weasel".
The other thing it does is at some point engage a mechanism that releases the pressure on a small door on the top of the box. Underneath same is a folded, spring-activated doll of some type. The practical upshot of this is that either at a specific point in the song (the "Pop" in "Pop Goes the Weasel", for example) or even more evilly, at some random point - the winder is surprised by having something literally explode into view, startling the winder.
This is a toy given to small children, which if you think about it is a prank being played on said child. The unsuspecting baby or toddler (the traditional age level at which this is given) winds the crank with the outer limit of his or her developing motor skills. The music lulls and rewards the unsuspectBANG didn't expect that did you?
In theory this plays on the concept of object permanence in a child. Things unseen don't exist. That's why peek-a-boo is so fascinating for a child - first you exist, then you don't. Your repeated appearance and disappearance is magic to a creature who cannot yet put together that you still exist when hidden.
But this rarely provokes that kind of whimsy in the vast majority of small children who are suckered into playing with it. The typical reaction to a noisy, banging interruption of their life with the sudden appearance of a threatening looking monkey or clown results in things like coulrophobia, crying, screaming, assault of the box, and so forth.
And the typical parental response to very understandable distress is laughter.
In fact, children who know of the trick at best will cautiously wind the thing, playing with their own fear in the same way adults will attend a horror film for entertainment. Or they will outright refuse to touch it and dissuade others from interacting with it, or run and hide, screaming.
So let's recap, there exists a toy whose only purpose in life is to trick trusting, small children into being suddenly startled and terrified for the amusement of adults, who will laugh at their pain. And this is given to them under the guise of something amusing and fun.
To me, this is unbelievably sick. Children that age know what laughter means on some level, and interpret it rightly as the parent deriving pleasure and happiness from their pain and their abuse of the child's trust.
Happily, children don't bother with toys anymore, so until someone comes up with scare videos for children on YouTube, the idea of hurting children as an amusing pastime will fade into obscurity.
For the record, that is the adult's version - a video, (especially one that advises you to turn up your speakers) that suddenly interjects a screaming, skull-like visage complete with loud terrified scream or other horrifying imagery and sound into an otherwise lulling video, or better yet one that requires you to really scrutinize a subtle detail. Adults don't seem to like the jump scare one bit, there's many a video of them spontaneously smashing or hurling the monitor against the floor or out a window. Never known an adult to enjoy the experience one bit.