As the pic shows tooth is covered by a protective layer called ENAMELapprx 3-4 mm thick. Below it is DENTINE layer which is yellowish n u can feel its painful existence when u have something cold or hot n u get a very tingling experience........this layer is richly supplied by nerve endings.The thinning of the enamel layer due to excessive grinding can lead to exposure 0f the dentinal layer increasing your sensitivity to heat and cold............
THE PICTURE SHOWS :: loss of enamel with exposed yellowish dentinal layer
Below this is ur blood rich and most important layer called PULP........any infection reaching this layer will cause pain n the scream"aah..oo...ooch".
HOW DOES BACTERIA HARM UR TEETH?
The food we eat contains healthy n unhealthy bacteria.....the harmful ones release toxins which have the capacity to dissolve the topmost layer of ur tooth ENAMEL.......proper brushing n mouthrinsing can prevent this.....
If not corrected they grow multiply on ur tooth surface and attack the next layer that is DENTINE...........forming a hole or cavity on ur tooth
HOW DO U DETECT A CAVITY AT HOME?
while brushing open ur mouth wide and check all ur teeth specially the back ones........any black deep marks which couldnot be removed by brushing can be a cavity so visit ur dentist ...
In this condition a filling is not sufficient so ur dentist may advice a root canal ...........