What is sharp, yet can never cut anything?

What silently reveals the passage of time in the mirror, unless constantly betrayed by the magic of the bottle? Solve the riddle → What makes a bird light, but also makes a clock wound? Solve the riddle → What kind of song do fish sing when they're having surgery? Solve the riddle → What first appears the day after, takes over your muscles, and refuses to move out? Solve the riddle → What can you go on, but also shows which way you take? Solve the riddle →