by Slightly Magic » Sep 11th, '07, 13:41
Here's what I do:
I set the whole thing up as a demonstration of a legitimate and documented phenomenon, rather than a "trick" (I would never mention the OOTW title). I tend to play it straight.
I also set up rather muted expectations re. the outcome. Something like "no one really knows why it happens, but there is a slight tendency to get significantly more than half right". That way, they're only expecting a slight (if any) bias towards correct guesses.
Half-way through, say something like "OK. Whenever this has been tested, it is always found that there is a slight tendency to favour the right-hand pile for red cards, possibly because of the close association between the words 'Red' and 'Right'. The way experimenters have got around this in the past is by getting the subject to reverse the order of the piles half-way through. It probably wouldn't affect our results that much, but let's do it anyway, just to be sure". I then place each marker card on top of the opposite pile, and ask the spec to carry on. You could get them to re-shuffle at this point if they want to, as an extra convincer that the cards are well-mixed.
In my (limited) experience, the spec does not suspect that you have an ulterior motive for switching.
To clean up at the end, continue the quasi-scientific tone. "Right. Let's count up the ones you thought were reds. Could you do these?" Hand them the 'red' packet from the 'correct' pile. Once they start to count them, you've got ample cover to be able to pick up the 'black' packet from the 'wrong' pile (which, of course, is actually full of reds) and count them. Once all the cards are counted, say something like "I've got 13 here. How many have you got? 12? OK, so that's 25 in total. Exactly half of the pack, in fact, if we discount the two markers. Let's put them together, and do the same for these - you thought these were all black, didn't you? There should be 25 again - let's check quickly though..." You could then pick up a pile and say "there's 25 cards here, which you've guessed are red. How many reds would there actually need to be to convince you that there might be a real ESP phenomenon going on?" That way, you've got a reason for counting up the cards, an action which (in reality) was only needed to provide misdirection for the switch.
Sorry for the waffle. I really like this trick.