根据短文内容填空。

     My grandfather is already sixty years old. But he has only fifteen birthdays. Do you know why?

When is his birthday?

    Keys: His birthday is on February 29th. It's a leap(闰年)year every four years. So he has only

fifteen birthdays.

1．An interesting______.

2.  My grandfather is______years old.

3.  He has only______birthdays.

4.  His birthday is on______

5.  A leap year happens(发生)every ______ years.