Bubba had long heard the stories of an amazing family tradition. It seems that his father, grandfather and great-grandfather had all been able to walk on water on their 21st birthday. On that special day, they'd each walked across the lake to the bar on the far side for their first legal drink.
So when Bubba's 21st birthday came a round, he and his pal Jim Bob took a boat out to the middle of the lake, Bubba stepped out of the boat .... and nearly drowned! Jim Bob just barely managed to pull him to safety.
Furious and confused, Bubba went to see his grandmother. "Grandma," he asked, "it's my 21st birthday, so why can't I walk 'cross the lake like my pappy, his pappy, and his pappy before him?"
Granny looked deeply into Bubba's troubled eyes and said, "Because your father, your grandfather and your great-grandfather were born in January, when the lake is frozen, and you were born in July, you dumb ass!"