Asked by: Jeremy Parker, Bromsgrove
Teeth are the hardest structures in the human body, but as a direct consequence of this, they are also uniquely vulnerable.
Tooth enamel is hard because it is almost entirely composed of an inorganic mineral called hydroxyapatite (a form of calcium phosphate). But acids can attack inorganic minerals, both those directly present in fruit and those produced by the bacteria in our mouths as they digest sugars. Worse still, enamel cannot be regenerated once the tooth has formed. This means that any damage to the enamel is cumulative over your lifetime.
Brushing your teeth regularly helps keep mouth pH from dropping too low and reduces bacteria. Washing the outside of your body is more about removing dirt, pheromones and bacteria rather than preventing damage to the skin itself. Skin is mostly impervious to these substances and is constantly sloughing away and regenerating itself from below anyway.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA