Simple riddles for teens with simple answers

Each morning I appear to lie at your feet. All day I will follow no matter how fast you run, yet I nearly perish in the midday sun.

You measure my life in hours and I serve you by expiring. I'm quick when I'm thin and slow when I'm fat. The wind is my enemy.

I am pronounced as a one letter alphabet. I am written with three letters. I am complete with just two letters. I am double. I am blue I am black I am brown I am gray I am yellow and I am green. I can read from both ends. I appear the same every way! What am I?

My life can be measured in hours. I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.

Born of water, I drown in water. A blood-thirsty beast you can barely see. What am I?

Why is a river rich?

Never trust this fellow’s grin. His teeth are sharp and course, his blood’s as cold as icicles, and he’ll show no remorse! What is it?

If will follow you for 1000 miles but not miss home. It desires neither food nor flowers. It fears not water, fire, knives, nor soldiers. But it disappears when the sun sets behind the western mountains. Who Am I?

This very thing you were born with pleases us all. It's even capable of making men fall, while only experienced by few it's treasured by all.

A mirror for the famous, but informative to all. I'll show you the world, but it may be a bit small. What am I?