Here's why:

* Sediment Definition: Sediment is any loose, solid material that is transported and deposited by wind, water, ice, or gravity.

* Shell Formation: Shells are made of calcium carbonate, a mineral.

* Weathering and Erosion: Over time, shells break down through weathering and erosion, creating smaller pieces.

* Transportation: These shell fragments are then transported by various agents (like waves, currents, or wind) and deposited in new locations.

Therefore, bits of shells fit perfectly into the definition of sediment. They are loose, solid materials that are transported and deposited, often forming important components of sedimentary rocks.