"Mysterious" is a word we use when something is difficult to understand, explain, or identify. It's often used for things that make us curious or confused because they are not known or clear. For example, if you found a hidden door in your house that you never knew about, that would be considered mysterious.
Which of these examples is the odd one out?
He received a mysterious phone call in the middle of the night, but the caller did not speak
The ending of the book left me wondering about the mysterious character who only appeared in the final chapter
His intentions were as mysterious as the full moon on a cloudless night
The woods behind our house are mysterious; sometimes we hear strange noises coming from them late at night
The girl always carried a mysterious box with her; she never shared what was inside it
