Answer:
I have a straight back and sharp teeth to cut objects.
I tell people to be careful when handling boxes.
I am the beginning of eternity the end of time and space the beginning of every end the end of every place.
I am the kind of dog that chases anything red. What am I?
I have three heads. Cut off one I become stronger. Cut off two I become ten.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.