They were given the name because they have three parts to their bodies.
There are many types of trilobites but they all have the same basic structure.
The cephalon (head) contains two amazing eyes.
The eyes are made of many lenses, each lens is a crystal of calcite - calcium carbonate.
There is a problem with calcite crystals.
Looking through in some directions, they split light to make two pictures.
Trilobite eyes have the crystals lined up in the one direction to avoid that problem.
Trilobite biscuits make an unusual idea for a kid who likes fossils.