If you have dogs, young children, or irresponsible adults that
know how to open a sliding door but not close it, this
brilliant, $3.50 DIY is for you.  Hook up a bottle of water or
sand to a small pulley with some nylon string, and voila! You've
got an automatic sliding door, as the video above demonstrates.
Your mileage may vary, naturally, depending on how stubborn your
door is at sliding closed.  Still, it's a great time- and
effort-saving invention, which might work with other kinds of
sliding doors. It was thought up by YouTube user clanmcarthur
and shared by his son, Reddit user Defckon.  DIY Automatic Patio
Sliding Door | YouTube via Reddit