From the ArcaMax Publishing, Jokes Newsletter:
http://www.arcamax.com/news/jokes/s-22825-714127
A boy from Duluth, Minnesota named Lars had heard stories of an
amazing family tradition. It seem that his father, grandfather and
great grandfather had all been able to walk on water on their 21st
birthdays, to the boat club across the lake for their first legal
drink.
So when Lars' 21st came around, he and his pal Sven took a boat out to
the middle of the lake. Lars stepped out of the boat and nearly
drowned. Sven managed to pull him to safety.
Furious and confused, Lars went to see his grandmother. "Grandma," he
asked, "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 into Lars' eyes and said, "Because your father,
grandfather and great-grandfather were born in January. You were born
in July.