If eleven plus two equals one, what does nine plus five equal?

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?

When Jack was six years old he hammered a nail into his favourite tree to mark his height. Ten years later at age sixteen, Jack returned to see how much higher the nail was. If the tree grew by five centimetres each year, how much higher would the nail be?

I welcome the day with a show of light, I stealthily came here in the night. I bathe the earthy stuff at dawn, But by noon, alas! I'm gone.

When I got four, I have it none.
When I got two, I have it some.
When I got none, I have it all.

What is it?

Give it food and it will live; give it water and it will die.

We are a group of eight brothers. We consider the weak, yet we protect the king in every battle. If we move ahead, we never turn back. Who are we?

What has no start, end, or centre?

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

As a whole I am both
safe and secure.

Behead me and I become
a place of meeting.

Behead me again and I am
the partner of ready.

Restore me, and I become
the domain of beasts.

What am I?