What is hidden in 3D image?

At a family gathering, Fred's father, grandfather, and great-grandfather bragged that they had all been able to walk on water to the bar across the lake for their first legal drinks. So when Fred's 21st birthday came around, he rowed out to the center of the lake, stepped out of the boat, and nearly drowned. Fred climbed back in and went to see his grandmother.

"Grandma," he said, "it's my 21st birthday, so why can't I walk across the lake like my father, his father, and his father before him?"

Granny looked kindly into Fred's eyes and said, "Because they were all born in January, and you were born in August."

Joke found on https://www.sysnative.com/ on Ongoing Joke Thread forum, posted on Jun 6, 2013 by DonnaB