I'd guess it refers to this:
>Picture an empty wine bottle with a cork secured at the top in the >usual way.
>Inside the bottle a metal ring hangs suspended by a string attached to >the cork.
>How is it possible to make the metal ring drop to the bottom of the >bottle without touching the ring, the thread, the cork or the bottle?
Alright, use a magnifying glass to focus the sun's rays on the thread, burning it in half and the ring will drop to the bottom.
