Bob, the goober, heard a rumor that his father, grandfather and great-grandfather had all walked on water on their 21st birthdays.  So, on his 21st birthday, Bob his good friend Brian headed out to the lake.  "If they did it, I can too!" he insisted.

When Bob and Brian arrived at the lake, they rented a boat and began paddling.  When the got to the middle of the lake, Bob stepped off of the side of the boat...  and nearly drowned.  Furious and somewhat shamed, he and Brian headed for home.

When Bob arrived back at the family farm, he asked his grandmother for an explanation.  "Grandma, why can I not walk on water like my father, and his father, and his father before him?"

The feeble old grandmother took Bob by the 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."