Here's one that's impossible for any JW to answer today. It proves that the F&DS doctrine is false.
Ask the elders if they would read a Bible passage and comment on it. Don't let on at first that you're going to be tackling the F&DS question, or they'll refuse to play along and make excuses. Have them first read Matthew 24:36-44 (NIV here):
"36 "No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father. 37 As it was in the days of Noah, so it will be at the coming of the Son of Man. 38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39 and they knew nothing about what would happen until the flood came and took them all away. That is how it will be at the coming of the Son of Man. 40 Two men will be in the field; one will be taken and the other left. 41 Two women will be grinding with a hand mill; one will be taken and the other left. 42 "Therefore keep watch, because you do not know on what day your Lord will come. 43 But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into. 44 So you also must be ready, because the Son of Man will come at an hour when you do not expect him."
Ask them exactly when "the Son of Man" will come. They'll answer that they don't know, since the event is yet future and is tied to the coming of "the great tribulation" and Armageddon. That's what you want to get them to admit -- that the "coming" is in the future. Therefore the setting of the passage is in the future. This has been Watchtower teaching for about 30 years. Next make a brief comment that verses 45-46 mention "the faithful slave", but you want them first to read Matthew 24:48-51:
"48 But suppose that servant is wicked and says to himself, 'My master is staying away a long time,' 49 and he then begins to beat his fellow servants and to eat and drink with drunkards. 50 The master of that servant will come on a day when he does not expect him and at an hour he is not aware of. 51 He will cut him to pieces and assign him a place with the hypocrites, where there will be weeping and gnashing of teeth."
According to present WTS teaching, the events described here are also in the future. Get the elders to admit this, too. You can look up the latest Watchtower comments in the WT-CDROM. Finally have them read the most fundamental passage that exists for JWs, Matthew 24:45-46:
"45 "Who then is the faithful and wise servant, whom the master has put in charge of the servants in his household to give them their food at the proper time? 46 It will be good for that servant whose master finds him doing so when he returns."
Don't let the elders comment yet, but you immediately point out that it's obvious that the passages immediately before and after the above are yet in the future, so logically this passage must describe future events. Why then, does the Society teach that it had a fulfillment in 1919? They will not be able to answer. You pounce on this and state that this is proof that the leaders of the Watchtower were not "appointed over all Christ's belongings" in 1919, and therefore they are teaching a great falsehood, which makes them false teachers, and therefore -- by their own definition -- false prophets. It is grossly arrogant to take up for themselves a title like "faithful and discreet slave" when the Bible itself proves that this cannot be.
AlanF