Final answer:
Shaking your head 'no' is achieved through the rotation of the atlas around the axis at the C1/C2 joint, which is a pivot joint allowing for this movement.
Step-by-step explanation:
When you shake your head "no," you are performing rotation along the longitudinal axis of the C1/C2 joint. During this movement, the first cervical (C1) vertebra, known as the atlas, rotates around the dens, which is the upward projection from the second cervical (C2) vertebra, known as the axis. This rotational movement is possible due to the pivot joint that exists at the atlantoaxial joint, allowing for the side-to-side rotation of the head.