The boundary between the crust and lithosphere isn't a distinct, physical division like a layer cake. Instead, it's a gradual transition zone where the properties of the rock change. Here's a breakdown:

* Crust: The outermost, solid layer of Earth, made of relatively light rock (granite for continental crust, basalt for oceanic crust).

* Lithosphere: The rigid, outermost layer of Earth, encompassing both the crust and the uppermost part of the mantle.

The "boundary" is defined by the change in behavior of the rock:

* Crust: Behaves relatively rigidly, but can fracture and break under stress.

* Lithosphere: Remains rigid and strong, able to support tectonic plates.

* Asthenosphere (beneath the lithosphere): This layer is more plastic and deformable, allowing for tectonic plate movement.

Think of it like a sandwich:

* Crust: The bread slices on the outside.

* Lithosphere: The whole sandwich, including both the bread and the filling.

* Asthenosphere: The soft, gooey filling inside.

Here's the catch:

* The thickness of the lithosphere varies. It can be much thicker under continents and thinner under oceans.

* There isn't a sharp line separating the crust and lithosphere. The transition is gradual, with the rock becoming progressively hotter and more ductile as you move deeper into the mantle.

In summary, the "boundary" between the crust and lithosphere is not a physical boundary, but rather a transition zone where the rock changes its behavior from relatively rigid to more plastic. This transition is driven by changes in temperature and pressure with increasing depth.