If you put your lips together and blow, you might be able to whistle. People whistle when they are happy or bored. Sometimes they whistle to get attention. A whistle is used by officials at sporting events to get the attention of the players.
This first set of sentences shows how "whistle" is used as a verb:
Gary whistles while he works.
Ryan doesn’t know how to whistle.
Miriam’s dog will come to you if you whistle.
A tea kettle whistles when the water begins to boil.
Someone whistled at a pretty woman to get her attention.
These sentences use "whistle" as a noun:
Referees use a whistle during football games to manage the action on the field.
Mrs. Smith uses a whistle to call children back into the school when recess is over.
A good whistle can be heard from a great distance. (great distance = far away)