It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?

Living above a star, I do not burn
Eleven friends and they do not turn
I can just be visited in a sequence, not once or repeatedly
PQRS are my initials
Can you tell my name accurately?

I never was there, I am always to be.
You have never ever seen me, not ever you will.
Yet I am the confidence of everyone.

Who Am I?

I hide my treasure in the ground, my tail is big and fluffy. If you spot me in a tree, please don’t call me scruffy. What am I?

Replace the question mark with the number below given a simple math problem.

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

What is the word or phrase?

How can you make the following equation true by drawing only one straight line: 5+5+5=550 ?

What brings you down but never up?

You are presented with three boxes. One of them has a red ball inside and the other two have a black ball inside each of them. You are asked to pick up the one with red ball and you pick one. Now, one of the other boxes is opened and it is found to have a black ball.

You are presented with a chance to change your box with the one that is left closed. Will you change your box? Why or why not?