I am taken from a mine, and shut up in a wooden case, from which I am never released, and yet I am used by almost everybody. Pencil leadWhat goes round the house and in the house but never touches the house? The sunWhat is it that you can keep after giving it to someone else? Your wordWhat walks all day on its head? A nail in a horseshoeWhat gets wet when drying? A towelWhat comes once in a minute, twice in a moment, but never in a thousand years? The letter M