Logical Joke

Johnny 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 Johnny's 21st birthday came a round, he and his pal Jim Bob
took a  boat out to the middle of the lake. Johnny stepped out of the
boat  ...and nearly drowned!

Jim Bob just barely managed to pull him to  safety.
Furious and confused, Johnny 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 father, and his father before him?'
Granny looked deeply into Johnny's crossed lil  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 dumbass!'