i've read this twice before i got the joke

All of his life Roger from Cape Breton had heard 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 day, they'd walk across the lake to the boat club for their first legal drink.

So when Roger's 21st birthday came around, he and his pal Corky took a boat out to the middle of the lake. Roger stepped out of the boat and nearly drowned! Corky just managed to pull him to safety.

Furious and confused, Roger went to see his grandmother. "Grandma, 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 Roger's eyes and said, "Because your father, grandfather and great grandfather were born in January, you were born in July."