I'm told this was my grandmother's favorite joke...

A young couple go into a movie theater and take seats way at the back.
After a while, stifled giggles arise from their location. Then louder, uncontrolled giggles.
   Finally, an usher hears them and goes over to the area and leans over the young lady and asks.. "Pardon me, m'am... are you feeling hysterical"
..and she giggled back.."No...he's feeling mine!"