There is a lot more to interpreting and recognising faces than you might think. Forgetting people we used to know is not that uncommon, and is something we are in fact all guilty of. But next time you're standing in the supermarket having a conversation with someone you don't recognise but act like you remember them, take heed that the likelihood is that they will know you are lying. Lying is a lot harder than most people realise, and hiding the fact we are lying is harder still. This is because we all make brief, involuntary facial expressions when we try to conceal or repress something, such as trying to remember someone. These have been termed micro-expressions. So when someone you don't recognise starts talking to you, you should just confess straight away because your micro-expressions will probably give you away anyway.