First molars (6 year old molars), second molars (12 year old molars) and some bicuspids (or premolars) grow in with deep grooves on the chewing surface of the teeth as part of their normal anatomy. These grooves are impossible to clean because they are too narrow and deep for a toothbrush bristle to get into. In the past, the majority of these groovy teeth got fillings, usually within the first two years.
These teeth are no longer doomed to this fate. We now have a material, a sealant, that will flow into these grooves. When placing a sealant, we clean the grooves thoroughly, both mechanically and chemically, and fill the grooves with this flowable material to prevent the otherwise inevitable decay and subsequent filling.
Below is a short video showing a sealant being placed.