Answer:
My teeth are sharp, my back is straight, to cut things up it is my fate.
I know a word of letters three. Add two, and fewer there will be!
To cross the water I'm the way, for water I'm above. I touch it not and, truth to say, I neither swim nor move.
Sometimes black, sometimes white, I have veins but no blood
I usually wear a yellow coat. I usually have a dark head. I make marks wherever I go.
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?