Best riddles for teens

I have lot of memories, but I own nothing. What am I?

What room do ghosts avoid?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test it was a blank sheet of paper with a solitary question on it: "What is risk?" Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word! What did he write?

What happens when you throw a blue rock into the yellow sea?

What do you get if you add 3 to 300 five times?

What has roots as nobody sees, Is taller than trees, Up, up it goes And yet never grows?

A very vicious dog is tied to a tree. If you came close enough, you'ed lose your head. If he sees you, he'll follow your every step. Your basket ball rolls past the dog. If you are alone, how do you get your ball back with out being mutilated?

I am alive without breath and cold as death. I am never thirsty but always drinking. What am I?

I hiss. I can be poisonous. I am long. I have a forked tongue. What am I?

In a new Engineering Hostels they have 100 rooms. Ankit Garg was hired to paint the numbers 1 to 100 on the doors. How many times will Ankit have to paint the number eight ?