Walk On Water?

Paddy 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 18th
birthday.  

 On that special day , they'd each walked across the lake to
the pub  on the far side for their first legal drink.

So when Paddy's 18th birthday came 'round , he and his pal Mick, took a boat out to the middle of the lake.  Paddy stepped out of  the boat ....and nearly drowned!  Mick just barely managed to pull him to safety.

Furious and confused, Paddy went to see his grandmother.
"Grandma'' he asked, "It's my 18th birthday, so why can't I walk  'cross the lake like my father, his father, and his father before him?"

Granny looked deeply into Paddy's troubled  brown eyes and said , "Because your father, your grandfather and your great grandfather were all born in December, when the lake is
frozen, and you were born in August, ya dip  shit."