It is very easy to ask something which is technically correct, but is leaving your self open to a silly reply if you asked the wrong person. One particular case springs to mine, where a poor, unsuspecting member of the public had the following conversation with a friend of mine: Joe Public - Excuse me, Guard, Guard. Guard - Yes sir, how can I help you? J.P. - Can you tell me how long the next train will be? G - Certainly sir, approximately 600 feet sir. He did go on to explain to the poor man after wards that he was pulling his leg, and give the information that the gentleman wanted.