Igneous rock that cools very quickly when it forms may have a fine-grained or glassy texture. Here's why:

* Fine-grained texture: When magma cools quickly, mineral crystals don't have enough time to grow large. This results in a rock with tiny, often invisible crystals, giving it a smooth, fine-grained appearance. Examples include basalt and rhyolite.

* Glassy texture: If the cooling happens extremely fast, like when lava erupts into water or air, there's not enough time for crystals to form at all. This results in a glassy texture, often with a smooth, shiny surface. Obsidian is a classic example.

Let me know if you'd like to know more about different types of igneous rock textures!