I WONDER - How was glass formed naturally?

The video tutorial explains the natural formation of glass, specifically obsidian, which occurs when sand from an erupting volcano melts in lava and cools rapidly upon contact with water. It highlights the dark greenish-black color of obsidian and how its discovery led humans to experiment and eventually learn to manufacture glass.

What process occurs when volcanic lava comes into contact with water?

What did people begin to do when they found obsidian?