Visiting a pub is one of Britain's oldest forms of entertainment. The idea for the first public houses was brought to Britain thousands of years ago by the conquering Roman army. The first pubs served only wine, but after the discovery of hops   in the fourteenth century, pubs began to serve mainly beer, as they do today. Today, there are 61,000 pubs in the United Kingdom. One of the oldest, Fighting Cocks in ST. Albans, Herts, is located in a building that dates back to the 11thcentury. Modern pubs are often owned by English breweries   and serve only their owner's products. British pubs are required to have a license, which is difficult to obtain, and operate between 11 a.m. and 11 p.m. every day except Sunday, when they must close at 10:30 p.m. The drinking age in Britain is eighteen, but fourteen-year-olds may enter a pub unaccompanied if they order a meal. Children may enter a pub with their parents until 9 p.m., which lets families enjoy reasonably priced pub meals together and allows pubs to continue in their traditional roles as community centers. Customs in British pubs differ from those in American bars. In Britain, you must go to the bar to order drinks and food and pay for your purchase immediately, there is no table service. Bartenders are called "landlords" and "barmaids" and they do not expect frequent tipping. To tip a landlord or barmaid, it is customary   to tell him to "buy himself one". He or she may or may not take your tip on your offer. At the end of a visit, a polite customer will return his glass to the bar. This passage is mainly written to   _  .
give us some information about British pubs compare the British pubs and American bars tell us something about the history of British pubs let us know some rules in British pubs.