The kibitzer was determined not to let the joking experts take him for a ride once more. Too often South had told his opponents they needed reading glasses 'since they held their cards too far from their chests'. The 'victims' then would admit their complicity with a grin. But that was popular theatre, 'for the gallery', since they knew very well that the declarer had chosen the winning line of play for a purely technical reason, i.e. a 'necessary assumption'.
Soon the kibitzer got his chance to show his newly acquired knowledge.

E/NS		♠	A Q 6 2
		♥	A K Q J 10
		♦	8 5 4
		♣	J
		♠	J 7
		♥	8 7 5 3 2
		♦	A Q 7
		♣	7 6 5

¹ Easy does it, the double forced South to bid, so he may even have bid 1♥ on a three card suit

West led the ♣4, East won with the ♣Q and played the ♣10 back; West the ♣9.
Declarer ruffed in dummy and drew two rounds of trumps, East following suit once, then discarding the ♣8. Then declarer turned around and grinned at the kibitzer.
Feverishly the kibitzer tried to figure out what was happening. It was obvious another reading-glasses joke was about to be launched. Then he saw the light, and not a moment too soon, because at that same moment declarer played the ♠2 from dummy. For a moment East hesitated, then he played a small one. South inserted the ♠J, which won, and lost only two diamond tricks (in addition to the ♣Q at trick one), since the ♦K turned out to be wrong and he could not ruff out East's ♠K. So the contract was just made:

E/NS		♠	A Q 6 2
		♥	A K Q J 10
		♦	8 5 4
		♣	J
♠	9 8 4			♠	K 10 5 3
♥	9 4			♥	6
♦	K J 10 3			♦	9 6 2
♣	9 4 3 2			♣	A K Q 10 8
		♠	J 7
		♥	8 7 5 3 2
		♦	A Q 7
		♣	7 6 5

'Your partner keeps his cards too far from his chest as well', South told East. 'I saw the ♦K in his hand, somewhat unexpectedly by the way, so finessing in diamonds was pointless.'
And then, towards West: 'You need reading glasses as well, my friend.'
Again EW played their assigned sheepish parts with a great deal of animation, but the kibitzer loudly claimed he was not going to fall for it:
'Come on, you had no choice but to play like you did. The only danger is the ♦K being with West. After all, if East has that card, you cannot lose anymore than a club, the ♠K and the third diamond.'
After a triumphant glance at South, he continued: 'So you have to make a sort of necessary assumption that the ♦K is wrong, with West. In that case the ♠K is certainly wrong as well, with East: he needs it for his opening. And so it was.'
'If you start off with the diamond finesse, which looks logical - after all, you are in dummy - West will win and return the suit. You have to lose the lead to the ♠K and subsequently lose another diamond: down one.'
'By playing a spade from dummy, you put East in a fix. He did the best thing by ducking, allowing you to just make your contract. Had he won with the ♠K, you would have won any continuation, unblocked the ♠J and pitched the ♦Q7 on the ♠AQ: an overtrick.'
'But what if West had won the ♠J with the ♠K?' asked South, who was beginning to enjoy himself.
'You haven't paid attention: in that case East is sure to have the ♦K, in view of his opening.'
South seemed impressed: 'Well, well, you have learned. Let's see if you can handle this one.' Shortly afterwards the kibitzer was looking at a piece of paper with this problem:

W/EW		♠	A J 7 6
		♥	9 3 2
		♦	A 3 2
		♣	A K 5
		♠	K 10 9 8 4
		♥	10 8 5
		♦	K 5
		♣	4 3 2

¹ Weak two, six card suit, 6-10 HCP

The kibitzer saw he had a seemingly inescapable club loser, not to mention the trump queen. Well, let's first locate the ♠Q, he thought. It seemed logical to play the ♠A first and next, if the ♠Q had not come down, to finesse East for that card. After all, there were more 'open spaces' in East's hand: he only had one heart, while West had six.
'I win with the ♣A and play the spade a...'
He stopped for a few seconds and then resumed: 'I play a spade to the king.'
'EW follow suit with small ones.'
'I play a spade from South and if West follows suit with a small one, I finesse by inserting the ♠J.'
'You succeed, East discards a diamond.'
'I draw West's ♠Q with the ♠A, cash the ♣K and then play the remainder of the trumps. On the last one I discard dummy's last club. I watch closely how many clubs are discarded. If my ♣4 is the thirteenth club, I cash it and then make the last two tricks with the ♦AK. If my club is not good, I play the ♦K and ♦A and hope dummy's ♦3 wins the thirteenth trick.'

It was obvious that the four players were impressed by the kibitzer.
'Er, yes, you made the contract, well played.'
'I don't get it', said the dummy. 'Why did you finesse for the ♠Q over West, against the odds?'
'Because it was my only chance', the kibitzer replied. 'This was the layout'.

W/EW		♠	A J 7 6
		♥	9 3 2
		♦	A 3 2
		♣	A K 5
♠	Q 5 2			♠	3
♥	A K J 7 6 4			♥	Q
♦	10 9			♦	Q J 8 7 6 4
♣	10 9			♣	Q J 8 7 6
		♠	K 10 9 8 4
		♥	10 8 5
		♦	K 5
		♣	4 3 2

'You haven't answered my question about the finesse for the ♠Q over West.'
'The only way to escape the club loser was this minor suit squeeze - a Vienna Coup* by the way - against East. In order for the squeeze to succeed East had to control the third round of both minors, since if West controlled one of them, East could discard in that suit.'
'Therefore the necessary assumption was that West had two cards in each minor at most. So East had to have 6-5 in diamonds and clubs. He had shown up with a singleton heart. Ergo: íf I could make my contract East had to have one trump at most. The chance of this distribution occurring was very slim indeed, but as I said before: it was my only chance.'
After briefly enjoying the looks of surprise all around him, he resumed: 'Of course, you lot would have started talking about those reading glasses again, claiming you had seen the ♠Q in West's hand. Very childish.'

He left the room, looking rather haughty.
'Still, that boy is his own worst enemy', South remarked philosophically.
'How do you mean?'
'Do you know what he does for a living?'
'...'
'He is the manager of the optician's shop nearly opposite this club.'

(*Typical for a Vienna Coup is the cashing of blocking honour cards first — here: the ♣AK — in order to 'liberate' the menace in the opposite hand — here: South's ♣4).