Here's why:

* Extrusive means the magma erupted onto the Earth's surface, where it cools rapidly in contact with air or water.

* Igneous means the rock is formed from cooled magma or lava.

Fast cooling leads to smaller crystals, resulting in fine-grained or glassy textures. Some examples of extrusive igneous rocks include:

* Basalt: A dark, fine-grained rock formed from the rapid cooling of lava flows.

* Rhyolite: A light-colored, fine-grained rock formed from the rapid cooling of acidic lava flows.

* Obsidian: A volcanic glass formed when lava cools so quickly that crystals don't have time to form.

* Pumice: A light, porous volcanic rock formed when gas bubbles are trapped in lava during rapid cooling.

These rocks are often characterized by their small crystal sizes, glassy textures, and sometimes even gas bubbles or vesicles.