There are so many people that the tickets are all sold out !   Luckily we got the last two .   Look , Danny . The Four Great Inventions !   Gunpowder , compass , the paper making , and the printing .   I really admire those who made such great inventions !   May , I don't see why they are great . They are just simple things .   They look simple today . But they were milestones in their times .   Milestones ? May , you must be kidding .   Just think Bill Gates , those inventions are as important as the windows system .   Well , I suppose the ancient inventors made a lot of money out of them .   No , quite the contrary .   Why ? Didn't you say that they were milestones ?   Yes , but they went to the public without any patent protection .   Well , their inventors were not as clever as Bill Gates . 