I go up, Though sometimes I am down. I can write, But I don't make a sound. I am the touchstone for lightness, But often I am black. I can help you to hit the sack.

This riddle cleverly points to the humble feather. Its ability to "go up" is evident in the flight of birds, where feathers are essential for lift and maneuverability. Yet, when shed or fallen, a feather can certainly be "down" on the ground. The clue "I can write, But I don't make a sound" refers to the historical use of feathers as quill pens, which were the primary writing instrument for centuries before modern pens. These quills allowed for silent inscription, unlike the scratching of a pencil or the click of a ballpoint.

Feathers are renowned for their incredible lightness, making them a "touchstone for lightness." Despite this, they come in a vast array of colors, including deep black, as seen in many bird species like crows or ravens. Finally, the phrase "I can help you to hit the sack" alludes to feathers' widespread use in bedding. Soft and insulating, feathers are a popular filling for pillows, duvets, and mattresses, providing comfort for a good night's sleep.

Beyond these riddle clues, feathers are fascinating biological structures. They are made of keratin, the same protein found in our hair and nails, and are unique to birds. They serve multiple vital functions, including flight, insulation to regulate body temperature, waterproofing, and even display for mating rituals. Their intricate structure, with a central shaft and interlocking barbs and barbules, makes them both strong and incredibly lightweight, a true marvel of natural engineering.