You might have heard of this trick before, but take note — it’s literally a lifesaver. Retinoblastoma is a rare type of aggressive eye cancer that almost exclusively affects young children. It develops as a tumor in the retina but can be diagnosed with just a smartphone. Children who have the disease often have a white glow around their pupils that shows up when photographed with a flash — and it's this fact that recently saved the life of Julie Fitzgerald's two-year-old son, Avery.

"It saved our son's life."

"I just had this gut feeling in my stomach that something was wrong with his eye," Fitzgerald told ABC News. She'd seen odd spots in Avery's eyes in photos, but after reading a viral story about an identical diagnosis, she took her son to the doctor who confirmed the presence of cancer. Avery unfortunately lost his eye to the disease but no more than that. "It turned out to be our worst nightmare but it saved our son's life," Fitzgerald says.

Mom catches 2-year-old son's eye cancer after seeing story on Facebook about it: http://t.co/6ICKcLf4of pic.twitter.com/M34seq3M2X — ABC News (@ABC) May 11, 2015

Spotting a white flash in a child's pupil isn't a definitive diagnosis of retinoblastoma, but it's always worth following up, say doctors. "It's much better to do an exam that has negative findings than to pass up an exam and not have it looked at," Mayo Clinic ophthalmologist Michael W. Stewart told USA Today. "When caught early [retinoblastoma is] very treatable. Current technology and chemotherapies can save the baby's life and vision."

As a diagnostic tool, smartphone cameras are so effective that the Childhood Eye Cancer Trust (CHECT) even ran an ad campaign last year centered on the trick. They put up posters of children's eyes that flashed white when photographed thanks to the use of reflective ink. If you want to know more then CHECT has a good online guide about how to spot the white glow as well as other symptoms of retinoblastoma.