In the early stages of dental decay there are no symptoms, but your dentist may be able to spot an early cavity when they examine or x-ray your teeth. This is why you should visit your dentist regularly, as small cavities are much easier to treat than advanced decay.
Decay happens when sugars in food and drinks react with the bacteria in plaque, forming acids. Every time you eat or drink anything containing sugars, the bacteria reacts with it to form acid. These acids attack the teeth and start to dissolve the enamel. The attacks can last for an hour after eating or drinking, before the natural salts in your saliva cause the enamel to 're-mineralise' and harden again.
It's not just sugars that are harmful: other types of carbohydrate foods and drinks react with plaque and form acid. (These are the 'fermentable' carbohydrates such as the 'hidden sugars' that can be added to processed food, natural sugars like those found in fruit, and cooked starches).
Snacking on sugary or acidic foods and drinks can increase the risk of decay, as the teeth come under constant attack and do not have time to recover. It is therefore important not to keep snacking on sugary foods or sipping sugary drinks throughout the day.