This riddle says: A man is 100 years old but has only had 25 birthdays. How is that possible?
The answer is: he was born on February 29.
February 29 appears only in leap years, which normally occur once every four years. Someone born on that date can celebrate the exact calendar date of their birth far less often than someone born on most other days.
Over roughly 100 years, February 29 would occur about 25 times, which is the idea behind the riddle.
Of course, the person still gets older every year and can celebrate a birthday on February 28 or March 1 in non-leap years. The puzzle is using “birthday” to mean the exact date February 29 appearing on the calendar.
That small distinction is what makes the answer work.
