Answer:
One simple click, one simple flash. A piece of memory, for years I'll last. What am I?
What word has kst in the middle, in the beginning, and at the end?
Curtail me thrice I am a youth; behead me once a snake; complete I'm often used in truth when certain steps you'd take.
What is shaped like a box, has no feet, and runs up and down?
I have leaves on my fruit my fruit is on my leaves.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.