Guess what! A UFO flew over my head while I was walking toward the bus stop yesterday.
Really? WHat did it look like?
It looked like a plate.
It may be a UFO.
It must be. I'm quite sure there are UFOs.
I'm not so sure. Even scientists aren't sure whether there are UFOs.
It can't be a UFO. You see, there are no living things on other planets.