answer: #Person2# says it's easy to tell whether it's someone's 21st birthday in America since people are allowed to drink at 21, and #Person1# says there're some traditional celebration ways in China but now it's no different from America.
documents: #Person1#: Look! Someone is celebrating his birthday. 
  #Person2#: This must be his 21st birthday. No doubt about it. 
  #Person1#: Why? Do you know that guy? 
  #Person2#: No. Well, in America, 21 is the age when you're allowed to drink. So, many guys celebrate it in bars. 
  #Person1#: That's interesting. But it would be really expensive, I suppose. Just think about all the drinks. 
  #Person2#: No. If it's your birthday, then you don't need to pay a cent. Just blow out the candles and open the gifts. That makes the birthday a special day. Your friends will take care of everything. 
  #Person1#: Wow. That's wonderful. 
  #Person2#: What about in China? What'd you do to celebrate your birthday? 
  #Person1#: Nowadays, it's no difference from America. But in old times, we have traditional ways to celebrate it.