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 Am I?

Why this works

This riddle plays cleverly with the word "snake" through various transformations. The first line, "Curtail me thrice, I am a youth," refers to the word "snake" being reduced to "s," which phonetically sounds like "ess," a term sometimes used informally to refer to a young person or youth. The second line, "Behead me once, a snake," indicates removing the first letter from "snake," leaving "nake," which isn't a word but hints at the original. The final line connects the full word "snake" to the idea of "certain steps you'd take," as snakes move in a slithering manner. Thus, each transformation leads us back to the word "snake" itself, making it the perfect answer!