Here's why:

* Size: Clay particles are the smallest, typically less than 0.0039 mm in diameter.

* Formation: Clay minerals are formed through the chemical weathering of rocks, often in the presence of water.

* Properties: Due to their small size, clay particles have a high surface area and can easily be transported by wind or water. They can form cohesive sediments like mud.

While silt particles are also very fine, they are slightly larger than clay.

Here's a quick comparison of sediment particle sizes:

* Boulder: > 256 mm

* Cobble: 64 - 256 mm

* Pebble: 2 - 64 mm

* Sand: 0.0625 - 2 mm

* Silt: 0.0039 - 0.0625 mm

* Clay: < 0.0039 mm