Tooth Decay - Caries in tooth

What - You can see small pits or black lines or visible hole on any surface of a tooth, sometimes part of tooth is even fractured if cavities are really deep.

Why - It usually happens due to accumulated food and bacterial action on it which causes acidic byproducts which further dissolves calcium of tooth surface involved and a hole is created which increases in size over a period of 3 to 5 years.