A sudden noise outside the door that the character can't explain. For example, in a haunted house story, the main character hears a scratching sound at the door, and just as they are about to open it, the story ends. It leaves the reader wondering what's out there waiting to come in.
One could be a character looking into a mirror and seeing something behind them that is not visible when they turn around. At the end of the story, they are about to look back again, leaving the reader on edge, wondering what it is that's lurking there.
One good cliffhanger could be when the main character is about to open a mysterious door and all you hear is a strange noise from behind it. The reader is left wondering what's on the other side and if it's something dangerous or amazing.
You can end a chapter or section with a sudden revelation that raises more questions. For example, a character finds out that their best friend has been lying to them all along, but you don't say why. This makes the reader curious and eager to find out the reason behind the lie.
One could be the main character suddenly hearing a strange noise right outside the supposedly safe room they've barricaded themselves in. It leaves the reader wondering if the source of the noise will break in and harm the character.
One cliffhanger could be the main character suddenly seeing a pair of glowing eyes in the darkness just as they think they are safe. Another could be a door slowly creaking open on its own when the protagonist has just started to relax. And a third could be the sound of a strange chanting starting up again just as the sun is about to rise.
In 'The Girl on the Train', the cliffhanger at the end of the book where the truth about what really happened on the night of the murder is only partially revealed. It keeps you guessing about who is truly responsible and what other secrets are still hidden. Also, in 'A Tale of Two Cities', Sydney Carton's self - sacrifice at the guillotine is a huge cliffhanger. You're left thinking about his final act of heroism and the impact it will have on the other characters.
In 'The Hunger Games' trilogy, at the end of the first book, when Katniss and Peeta are both declared winners but the Capitol seems to be up to something. It makes you wonder what will happen to them next. Also, in 'Harry Potter and the Goblet of Fire', the end where Voldemort has been resurrected and Harry has just escaped. It sets up a whole new level of danger and mystery for the subsequent books.
Cliffhangers in series novels serve to keep readers hooked. They create suspense at the end of one book, making readers eager to pick up the next in the series to find out what happens next. For example, in a mystery series, a cliffhanger might leave the main character in a dangerous situation, like being trapped in a room with a suspected killer. This makes the reader want to know if they'll escape.