Ben: Guess what! I know something you don’t know!
Layla: What’s that?
B: How many planets are there in the solar system（太阳系）?
L: That’s easy. Everyone knows that there are nine.
B: Not anymore! Can you believe it? They’ve decided that Pluto is not a planet anymore!
L: Nice try. I wasn’t born yesterday, you know.
B: I’m serious. They’ve decided that it’s too small to be a planet, but actually（实际上） they haven’t agreed on how big something has to be in order to be a planet anymore.
L: That sounds crazy. They can’t just change their mind about things like that.
B: Yes, they can. If you remember correctly, people used to believe that the world was flat.
L: 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.
B: We actually know relatively little about space and the cosmos.
L: Do you think that we’ll one day be able to travel to another planet for a vacation?
B: I suppose we could actually live on a planet outside of the Milky Way.
L: Do you think we’ll ever get to meet an alien from outer space?
B: I hope not. I think they would be a threat to those of us that lived on the Earth.
L: You’re so old fashioned. That’s what people used to think about people from another country!
B: Point taken. Hopefully one day, we’ll live in an interplanetary society.