1. Do you believe him?

2. Do you believe it?

3. I think it's hard to believe.

4. It's too good to be true.

5. I don't buy your story.

6. Believe it or not.

7. Really?

8. Are you kidding me?

9. You must be joking.

10. Are you joking?

11. Are you serious?

12. I don't think so.

13. I doubt it.

14. I doubt that he likes you.

15. I am doubtful about that.

16. It's doubtful whether this will work.

17. Don't expect me to believe this.

18. It can't be ture.

19. How is that possible?

20. I never take him too seriously.

21. He is a doubting Thomas.

22. I am at a loss what to do.

23. I feel puzzled.

24. It's a hard nut to crack.

25. You got me.

26. What's to be done?

27. I'm in trouble.