Three mischievous Grandmas were sitting on a bench outside a nursing home when an old Grandpa walked by. 
  
One of the old Grandmas yelled out saying, 'We bet we can tell exactly how old you are.' 

The old man said, 'There is no way you can guess it, you old fools.

One of the old  Grandmas said, 'Sure we can! Just drop your pants and under shorts and we can tell your exact age.' 

Embarrassed just a little, but anxious to prove they couldn't do it, he dropped his drawers. 

The Grandmas asked him to first turn around a couple of times and to jump up and down several times. Then they all piped up and said, 'You're 87 years old!' 
 
Standing with his pants down around his ankles, the old gent asked, 'How in the world did you guess?' 

Slapping their knees and grinning from ear to ear, the three old ladies happily yelled in unison - - 
  
'We were at your birthday party yesterday!'