Age Potion

A couple received a letter from their
daughter who went to study modern
physics overseas, the letter read:
"My beloved Parents, I miss you so much
and it breaks my heart to think that by the
time I get back you'll be too old. So enclosed
you'll find a bottle of potion I have invented.
It will make you young, so when I return
you'll be the same age as I left you.
NOTE: Please take only a drop"
So they opened the envelope and in it there
is a bottle with a red potion. the man looked
at the wife and says: "You go first." (typical
of men!)
So the wife takes a drop thereafter, the
husband follows. Indeed the wife turn five
years younger. Years later the daughter
returns home to find her mother young and
pretty, carrying a baby on her back. The
mother proceeds to tell her daughter how
the potion worked and made her look
young.
The daughter was delighted and asks after
her dad.
MOTHER: Your father? Hmm, my child, your
father was so jealous that I was so young
and beautiful so he drank the whole bottle.
DAUGHTER: What? So where is he?
MOTHER: Hahaha, who do you think is the
baby on my back?