"Just because"

My sister was taking my dad, 82, to the urologist today because of a complaint of urinary frequency. My other sister asked him if this was a new problem. My older sister interjected "Yes...every 10 minutes...not that he has had to go during the two hours that we've been out."

My dad replied, "Just because I didn't ask to go to the bathroom, doesn't mean that I haven't gone."