Answer:
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
What has six faces and twenty-one eyes?
I pass before the sun, but make no shadow.
What is never used unless it's in a tight place?
What lies in a tunnel of darkness. That can only attack when pulled back?
What is so fragile that saying its name breaks it?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?