The Best of Two Worlds- Meaning when you enjoy two different world or opportunities at the same time then you get the best of two worlds. If you are working in office and still you take out time like me to write a blog then you get the best of two world. 

Example - Doing part time job  for two days a week and looking after his kids as a single father he gets the best of two worlds. 

speak of the devil-When you are talking about some person and that person enters in between then you use this phrase as speak of the devil and he is here. There are many times you speak of someone then you get appalled seeing him because you did not expected his presence at the most and you get confused without knowing how to react and you start murmuring slowly without knowing or analyzing what to say in front of that particular person. 

Example - Hello momin, speak of the devil, i was just telling my friend Roman about your new dress and look you are here. 

See eye to eye:- when you get agreed with the other person then you can see eye to eye. at some point when you get to understand the others problems and then you place yourself in his position then you realise that when you put yourself in his position then you come to know that you would have acted the same way if you were in his position and when you start understanding other person then you get the best clarity of understanding. 

Example - Those business man finally saw eye to eye after 6 months of confrontation. 

Once in a blue moon- An event that happens once in a while or  infrequently. suppose you go to the restaurant once in a 3 month then you visit restaurant once in a blue moon. When you have a exam next year then you will study only once in a blue moon that is before 1 week of your exam. You get married only once in a blue moon. 

Example - I only go for vacation once in a blue moon. 

When Pigs fly-When something you say is impossible or will never happen then you say Pigs fly. because as you know pigs cannot fly. Things that will never take place, remain as it is. 

Example - When pigs fly then she  will clean his room.