Tooth decay is caused by the bacteria found in dental plaque. In general, the longer dental plaque remains on a tooth's surface the more likely it will be able to form a cavity. The dental plaque that has accumulated on a tooth's surface is scrubbed off when a person brushes his teeth.
Some back teeth, especially molars, can be difficult for a person to clean because the grooves found on their chewing surface are deep and narrow. Even though the person brushes his teeth not all of the dental plaque that is present is cleansed off because the individual bristles of the toothbrush are simply too large to gain access into the depths of the tooth's grooves. Because some plaque has not been cleaned away, the tooth is at risk of decay.