Here's why:

* Erosion: Rocks that are least resistant to weathering (attack by water and air) are the ones that erode most quickly.

* Landscape Formation: Erosion is a key process in shaping landscapes. When less resistant rocks erode faster, they leave behind the more resistant rocks, often forming features like cliffs, mesas, or canyons.

Example: Imagine a landscape with both sandstone and granite. Sandstone is softer and weathers faster. Over time, the sandstone will erode away, leaving the harder granite standing tall.