The most common causes are dryness, nose picking and injuries.

PREVENTION
Keep the lining of your nose moist.
Keep children's fingernails short to discourage nose picking.
Quit smoking. Smoking dries out your nose and also irritates it.
Open your mouth when you sneeze.

SIGNS
Blood from the nose can be either dark or bright in color.
If you lose a large amount of blood, you may become dizzy.
If you swallow blood, your stools may turn black.

FIRST AID
Sit down.
Lean slightly forward to prevent blood from running into your throat.
Place cold, wet cloths on your nose to constrict the blood vessels in your nose and stop the bleeding.
If blood is coming from only one nostril, press firmly at the top of that nostril.
If both nostrils are bleeding, pinch your nostrils together for at least 10 minutes.
If bleeding continues, apply pressure for another 10 minutes.
If the bleeding is the result of direct injury to the nose, only gentle pressure should be applied.
If heavy bleeding persists or if nosebleeds recur frequently, consult a physician.

CALL IMMEDIATELY FOR EMERGENCY MEDICAL ASSISTANCE IF:
The bleeding goes on for more than 15 minutes.
The bleeding was caused by an injury.
You get nosebleeds often.