One explanation of humour is based on the fact that a great deal of humour is a consequence of language. Language is an approximation of thoughts through symbolic manipulation, and the gap between the expectations inherent in those symbols and the breaking of those expectations leads to laughter. Irony is explicitly this form of comedy, whereas slapstick takes more passive social norms relating to physicality and plays with them. In other words, comedy is a sign of a 'bug' in the symbolic make-up of language, as well as a self-correcting mechanism for such bugs. Once the problem in meaning has been described through a joke, people immediately begin correcting their impressions of the symbols that have been mocked. This is one explanation why jokes are often funny only when told the first time.