How do lie detector apps work?

Apple's app reacts on the basis of voice stress analysis. A traditional lie detector reacts to changes in temperature and moisture of the skin that occur when you are knowingly lying. Their weak point is that if someone is an 'easy' lier by nature, has started believing his own lies or mistakenly thinks that what he tells is the truth, the lie detector will not react. Also, if you have the time it is possible to 'train' for questions where you may want to lie, so the lie detector doesn't notice a skin or voice stress change. On the other hand very nervous people may cause 'false positives' even when they are not lying. It's why almost everywhere, lie detector results are no longer accepted as 'proof' by Courts. Apple sell its app just as fun and games, and rightly so.