Human babies first laugh at the age of around three or four months, usually as an expression of surprise. Babies' laughter often produces a positive response in adults who will involuntarily copy the child. Video footage, on the YouTube site, of babies laughing was shown to Queen Elizabeth II during her visit, on 16 October 2008, to the Google headquarters, where both she and her husband the Duke of Edinburgh were reduced to 'fits of giggles'.