Since we have only this short phrase 'and jenny's head fell off', we can only make wild guesses about the full story. It could be that this is a part of a supernatural story. For example, Jenny might have been cursed, and as a result, her head suddenly detached. Or perhaps it's a gory part of a horror - themed play or a short story written by someone with a very vivid and dark imagination. But really, without further details, it's impossible to tell the real full story.
I'm not sure specifically which 'Jenny's head fell off' story you are referring to. There could be various fictional or even some very strange real - life inspired stories with such a title. Without more context, it's difficult to provide the full story.
One possibility is that it's a story set in a post - apocalyptic world. Jenny was exploring an old, dilapidated building full of traps. She accidentally triggered one of the traps, which was a sharp blade that swung down and severed her head. Another idea could be that in a world of magic gone wrong, a spell backfired on Jenny, causing her head to be removed from her body in a rather gruesome way.
Once upon a time, there was an old, haunted house at the edge of town. A curious girl decided to explore it. As she entered a dark room, she saw a strange figure in the corner. Suddenly, its head just fell off and rolled towards her. She screamed and ran out of the house, never to return again.
Maybe in a serial killer's lair. You're tied up and can't move. The killer is showing you his trophies, which are the heads of his victims. Then, suddenly, one of the heads falls off right in front of you. The sight of the decapitated head rolling and the knowledge that you might be next is truly terrifying.
I'm not actually aware of a real story about a spring breaker biting the head off a hamster. It could potentially be a very disturbing and unethical act if it were true. It might also be some sort of made - up horror or shock story that circulates in certain extreme or fictional contexts.