Guess what ! I know something you don't know !   What's that ?   How many planets are there in the solar system ?   That's easy . Everyone knows that there are nine .   Not anymore ! Can you believe it ? They've decided that Pluto is not a planet anymore !   Nice try . I wasn't born yesterday , you know .   I'm dead serious . They've decided that it's too small to be a planet , but actually they haven't yet agreed on how big something has to be in order to be a planet anymore .   That sounds crazy . But they can't just change their mind about things like that .   Yes , they can . If you remember correctly , people used to believe that the world was flat .   I suppose you're right . They also used to think that they were so important that the sun revolved around them , not the other way around .   We actually know relatively little about space and the cosmos .   Do you think that we'll one day be able to travel to another planet for a vacation ?   I suppose we could actually live on a planet outside of the milky way .   Do you think we'll ever get to meet an alien from outer space ?   I hope not . I think they would be a threat to those of us that lived on the Earth . 