Pick the odd one out.
Elbow, knee, neck, skull.

Explanation

To determine the odd one out from the options provided (elbow, knee, neck, skull), we can follow these steps:

### Step 1: Identify the nature of each option
- **Elbow**: This is a joint that allows movement; specifically, it is a synovial hinge joint.
- **Knee**: This is also a joint that allows movement; it is classified as a hinge joint.
- **Neck**: The neck contains joints that allow for movement; it has a pivot joint.
- **Skull**: The skull is made up of bones that are fused together and do not allow for movement; it is classified as an immovable joint.

...