flying

Are you afraid of flying?

In this case "flying" is not flying like a bird, but rather traveling by plane.  Notice the following:

1. Are you flying to Miami or driving?
   
2. I am flying to Bogota early tomorrow morning.
   

opportunity

If I get the opportunity to fly I will be very happy.

In this example, "opportunity" can be replaced by "chance" or it is a good situation to do something.  Notice the following:

1. She has the opportunity to learn a lot in her job.
   
2. His boss gave him the opportunity to work with the company's most important client.
   

sure

Yeah, sure.

In the example, "sure" has a similar meaning to "of course" or "yes."  It is a word of agreement.  Notice the following:

1. Sure.  I'll help you study for your exam.
   
2. Sure.  We would love to come to your cabin this weekend.
   

each other

I've seen cobras fighting with each other.

"Each other" is a pronoun used, in this example, to describe the actions between one cobra and another.  They are both performing the same action, directed at the other cobra.  Notice the following:

1. They have liked each other since they were 13 years old.
   
2. We looked at each other across the restaurant.
   

sin

In India it's a sin to kill a cobra.

A "sin" is a moral or religious crime.  For example, killing another person is both a religious crime and a legal crime.  Notice the following:

1. Do you think telling lies is a sin?
   
2. It is a sin to steal from another person.