I would like to say that first of all,
.....there is NO SURE miracle subject or answer you can provide....that will guarantee a receptive ear.
One of the best ways I have seen to make a witness think about their own faith......is to take out the foundation......that God has chosen them alone as his mouth piece.
First you ask if you could quote from a few watchtower refrences that you have questions about. Using their own literature cannot be refuted by them.
Then you show refrences in the watchtower which claims god has had a VISABLE earthly class of annointed followers.....each feeding the next group spiritual food.....hence feeding off one another.....un-interupted since Pentecost, until now.
Then you show them the refrernce in the' thousand year book' and the 'JW's in the 20th century' old green book, in which it says that RUSSELL....learned his truths from NOBODY. God used him and turned the light on bible truths. He 'INDEPENDENTLY took the bible of the shelf and did not at it through sectarian glasses'....it says.
I have not seen this argument ever been won by a witness, or not had an impact.
Gumby