Three mischievous grandmas were sitting on a bench outside a nursing home.

An old man walked by, and one of the grandmas yelled out saying, “We bet we can tell exactly how old you are.”

The old man said, “There ain’t no way you can guess it, you old fools.”
One of the grandmas said, “Sure we can! Just drop your pants and we can tell your exact age.”

Embarrassed just a little, the old man dropped his pants.

The grandmas stared at him for a while, asked him to turn around a couple of times, asked him to jump up and down for a little while and then they all piped up and said, “You”re 91 years old!”

“How in the world did you guess?” the old man said in shock.

The grandmas snickered and laughed.
Slapping their knees and grinning from ear to ear, all three happily yelled in unison, “Because we were at your birthday party yesterday.”