A clock that works by the sun's shadows is called a sundial.

Here's how it works:

* Gnomon: A sundial has a vertical stick or triangular piece called a gnomon.

* Shadow: As the sun moves across the sky, the gnomon casts a shadow.

* Hour Marks: The sundial has markings on a surface (usually a flat plate) that correspond to different hours of the day. The shadow's position on these markings indicates the time.

Sundial design can vary greatly, with some being quite complex. However, the basic principle of using the sun's shadow to tell time remains the same.