Two Yanks touring London joke

Two Yanks touring London in a taxi. What is that asked one of the Yanks. Why that is Buckingham Palace answered the taxi driver. Well you should see the states we have much bigger houses over there, and that. That is the Post Office Tower. Oh our towers are much bigger. This went on for much of the day until they went past a another building. Our buildings are much bigger than that one too. I thought it might be said the taxi driver, That is the mental institute.