This is a well known tale about the use of the word 'obvious' in science and maths.
A mathematics lecturer is giving a lecture when one of the students ask how one line follow from the one above. 'It is obvious', replies the lecturer immediately. Then he stops...scratches his head for a while... and finally walks out of the lecture theatre.
Twenty minutes later he returns, says, 'Yes it is obvious', and continues the lecture. Martin Hogbin (talk)