The tooth crashed when you eat, repeatedly, a food acid such as lemon

The tooth crashed when you eat, repeatedly, a food acid such as lemon
or carbohydrates (sugars) in all foods.
The lemon acts directly on the tooth. He melted enamel.
Carbohydrates (sugars) are used by cariogenic bacteria (always present in the mouth) to develop within the plaque. These bacteria convert sugars into acid. The acid produced, melts the enamel.

Each contact with the acid, the enamel of the tooth undergoes demineralization. It merits. It loses the "calcium phosphates, which are released in saliva. But the tooth can recover through saliva, which returns to its phosphate enamel lost. This is called remineralization.
This demineralization / remineralization occurs permanently in your mouth when you eat. When demineralization takes over on the remineralization, known as cavities.
This happens when:
- Saliva is inadequate
- And / or consumption of carbohydrates and acidic products is too frequent
- And / or dental plaque is not removed.

Brush your teeth regularly and daily.

The plaque
The plaque is a cake (a package) of food debris and bacteria. It is a soft, white dépôt bonded to teeth that develops within hours. Within the plate, two kinds of bacteria develop.

Bacteria called "cariogenic" which feed carbohydrates present and turn it into acids that attack teeth and cause demineralization called cavities.
Bacteria called "parodontogènes" who are responsible for problems periodontal (gum and bone). Some people develop more cariogenic bacteria and other bacteria over parodontogènes. The plaque is more or less virulent among individuals. Hence the need for brushing that eliminates any risk. Without plaque, no cavities and no (or little) of periodontal problems. NO PLATE NO DENTAL CARIES.