As a noun:
* The sign above the door read "Closed for lunch."
* She gave a subtle sign to her friend, indicating she was ready to leave.
* The sign of the times is that people are increasingly using technology for communication.
* The signs of a bad cold are a runny nose and sore throat.
* There were many signs that the company was in trouble.
As a verb:
* I signed the contract after reviewing the terms.
* The band signed autographs for their fans after the concert.
* The artist signed the painting in the bottom right corner.
* The protesters signed a petition demanding change.
* He signed up for the marathon to challenge himself.
Figuratively:
* The sign of a true friend is someone who is always there for you.
* Her smile was a sign of her happiness.
* The storm clouds were a sign of the coming rain.
Remember, the meaning of "sign" can change depending on the context of the sentence.