walking on the lake

Mike heard a rumor that his father, grandfather and great-grandfather had all walked on water on their 21st birthdays. So, on his 21st birthday, Mike and his good friend Brian went out to the lake.
"If they did it, I can too!" he insisted. When they arrived at the lake, they rented a boat and began padding. When they got to the middle of the lake, Mike stepped off the side of the boat . . . and nearly drowned. Furious and somewhat ashemed, he and Brian headed for home. When he arrived back at the family farm, Mike asked his grandmother for an explanation. "Grandma, why can't I walk on the water like my father and his father, and his father before him?"
The feeble old grandmother took Mike by his hands, looked into his eyes and explained, "That's because your father, grandfather and great-grandfather were born in January . . . you were born in July, dear."