The difference between a pebble and a rock isn't always clear-cut, as it's more about size and context than a distinct geological definition. Here's a breakdown:

Pebble:

* Size: Typically refers to a small, rounded piece of rock, usually less than 2.5 inches (6.35 cm) in diameter.

* Formation: Pebbles are often formed by the erosion and weathering of larger rocks, getting smoothed and rounded by water or wind.

* Context: Pebbles are commonly found in gravel, riverbeds, and beaches.

Rock:

* Size: A rock can be any size, from small pebbles to massive boulders.

* Formation: Rocks are naturally occurring solid materials made up of one or more minerals. They are formed through various geological processes like igneous, sedimentary, and metamorphic processes.

* Context: Rocks are found everywhere on Earth, from mountains to deserts to ocean floors.

In essence:

* Pebbles are smaller, smoother, and often more rounded pieces of rock.

* Rocks encompass a wider range of sizes and shapes, including pebbles.

Think of it this way:

* A pebble is a small rock.

* A rock can be many things, including a pebble.

The distinction isn't always strict, and sometimes the term "rock" is used to describe any piece of solid earth material, regardless of size.