Answer:
I open wide and tight I shut, Sharp am I and paper-cut fingers too, so do take care, I'm good and bad, so best beware.
You can hold it without using your hands or your arms. What is it?
Crooked as a rainbow, and slick as a plate, Ten thousand horses can't pull it straight.
Without this, eating food and Steven Spielberg's career would be very different.
The more you take, the more you leave behind.
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?