An improvised technique for the HEADS or TAILS plot, (or else, as it can be done with any flat object, like an index card or a piece of paper for instance with a different information on each side (that can be different than HEADS or TAILS, and allow a more captivating story than a binary game, or a bet).  
Effect (as perceived by the audience):
« Please take a coin, toss it, and watch secretly which side is up»... « There is no way anybody here can know if you lie or tell the truth »... «Now, if you want, you can turn the coin over. You can do it covertly, as many times as you want. »
When the spectator is satisfied, you ask only one innocent question, and you correctly guess which side is up at the end. This effect can be repeated identically, with different questions to make it more non-backtrackable. (Only one question is needed for the OSTRAKINDA variant.)
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