I bend but never bow. I strike without a fist. I mark the spot where blows are taught. Yet feel them twice when missed. What am I?

Tickle my bones, and I'll begin singing, With each tap a tone, Each rib will be ringing

I come straight as an arrow, but I'm then twisted with care I'm metallic and thin as a strand of fine hair. I bind without glue, without punches, without tape My uses lie mostly within my bent shape.

I begin with a letter and end with a number when spoken out loud yet neither has anything to do with me. Rather, think of an animal you know well

A test without me can't be graded. A door without me isn't secure. If made of bone I can fit anywhere.