At my high school one week out of the year the school hands out a new riddle every day you have to answer it and find the object with the #5 taped on it. Here it is.

Sometimes I'm wooden I'm plastic or steel mostly I help but I've been known to kill.

I'm found in your head and I'm held in your hand in trucks and cars and homes and stores cross the land.

You watch with excitement and millions do too, on Saturday night use me as your clue.

I'm pre-neolithic I'm every day, I'm used at your work, with sports and at play.

I'm found in the ocean and most every sea, the world would be different if it weren't for me

Thanks for the answer,
I have an idea of what it is

Something keen I think like a knife.