on earth 究竟
特殊疑问句中加上"on earth"可以加强语气，表示强调，有很强的感情色彩，意思是“究竟……；到底……”。另外，也可以用"the hell; in the world"。
1. Why on earth do you tell a lie?
2. Who on the earth do you think you are anyway?
3. Where in the world did you find the lost watch?
A: I've been standing here since half past seven. Where on earth have you been?
B: I'm terribly sorry I'm late. I just couldn't help it.
A: It's really very easy saying you're sorry.
B: Look, just give me a chance. I can explain it.