* Molecule: The smallest unit of a chemical compound. An arm is far too complex to be a single molecule.

* Cell: The basic unit of life. While an arm is made up of cells, it's a much larger and more organized structure.

* Tissue: A group of similar cells working together. An arm is composed of multiple types of tissues (muscle, bone, connective tissue, etc.) working together.

* Organ: A structure made up of different tissues that work together to perform a specific function. The arm is designed to move, grasp, and manipulate objects, making it a functional organ.

Let me know if you have any more questions!