Here's why:

* Fast cooling means the molten rock (magma or lava) solidifies rapidly.

* This rapid solidification doesn't give mineral crystals enough time to grow large.

* As a result, the resulting rock has many small crystals that are difficult to see with the naked eye.

Examples of fine-grained rocks:

* Basalt: A common volcanic rock formed from the rapid cooling of lava.

* Rhyolite: A volcanic rock with a very fine-grained texture.

* Gabbro: A coarse-grained igneous rock formed from the slow cooling of magma. (Though gabbro is coarse-grained, it's worth noting that it's still formed from igneous rock and, if cooled very quickly, it can have a fine-grained texture.)

Contrast this with slow cooling:

* Slow cooling allows crystals to grow larger, resulting in coarse-grained rocks.

* These rocks have easily visible crystals.

* Examples: Granite, diorite, peridotite.