There are two definitions that are used to describe closed questions. A common definition is: A closed question can be answered with either a single word or a short phrase. Thus 'How old are you?' and 'Where do you live?' are closed questions. A more limiting definition that is sometimes used is: A closed question can be answered with either 'yes' or 'no'.By this definition 'Are you happy?' and 'Is that a knife I see before me?' are closed questions, whilst 'What time is it?' and 'How old are you?' are not. This causes a problem of how to classify the short-answer non-yes-or-no questions, which do not fit well with the definition for open questions. A way of handling this is to define 'yes-no' as a sub-class of the short-answer closed question.