Here are some examples:
* "The stop sign was red." (The sign itself is the noun.)
* "He gave me a sign that he was happy." (The sign is an action or gesture, functioning as a noun.)
* "There were signs of life on the planet." (Signs are evidence or indications, functioning as nouns.)
However, "sign" can also be a verb:
* "He signed his name on the document." (He performed the action of signing.)
So, the part of speech "sign" depends on how it's used in a sentence.