Three mischievous gr

Three mischievous grandmas were sitting on a bench outside a nursing home.  About then a grandpa walked by, and one of the old 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 ornery grandmas said, "Sure we can!  Just drop your undershorts and we can tell your exact age."Embarrassed just a little, he dropped his drawers. 

 The grandmas stared at him for a while, asked him to turn around a couple of times, then asked him to jump up and down for a little while and then they all piped up and said, "You're 84 years old!""How in the world did you guess?!?"The ornery old 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."