What common phrase is represented by the below lines

Easy going:
Weak, 'I'm going.'
Tough, 'I'm staying.'

Medium going:
Weak, 'I'm going.'
Tough, 'I'm staying.'

Tough going:
Weak, 'I can't do it, I'm staying!'
Tough, 'Let's get going.'

A shepherd had 18 sheep, out of which, all but ten died.

Can you tell us how many sheep was he left with?

What word begins and ends with an E, but has only one letter?

You have a basket containing ten apples. You have ten friends, who each desire an apple. You give each of your friends one apple.
Now all your friends have one apple each, yet there is an apple remaining in the basket.
How?

The captain of a ship was telling this interesting story: "We travelled the sea far and wide. At one time, two of my sailors were standing on opposite sides of the ship. One was looking west and the other one east. And at the same time, they could see each other clearly." How can that be possible?

John killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

A Child was born in Lahore, Pakistan. Still, the child is not a Pakistani citizen. why?

Take away the whole and some still remain. What is it?

They do not possess flesh or feathers or scales or bone
But still, they are blessed with fingers and thumbs of their own.

What are the next two letters in the following series and why?
W A T N T L I T F S _ _

*Hint: Check Puzzle Title