Those who receive treatment within three hours of having a stroke have a better chance of recovery, says Ralph L. Sacco, MD, director of the Division of Stroke and Critical Care at Columbia University in New York City. Here are three simple tasks to ask of a person if you think she's having a stroke. If she fails even one of them, call 911 immediately.

Do the smile test. Ask the person to smile. Strokes cause weakness or tingling on one side of the body, so a person having one may also have a crooked smile.

Follow it with the arm test. Next, ask her to raise both arms. "If she only raises one arm or doesn't understand you, that's a worrisome sign," says Dr. Sacco.

Ask her to repeat a sentence. Strokes can cause speech difficulties, so if she can't repeat something like, "It's raining," call for help right away.