Composite resin fillings are used to fill a tooth after decay is removed. The most obvious advantage over the old metal fillings is that it is tooth colored and virtually unnoticeable. More importantly, metal fillings expand and contract with temperature, slowly causing cracks in the tooth. This allows bacteria underneath the filling, causing recurrent decay that is difficult to detect. Sometimes, these small cracks become fractures, which can lead to serious damage to the tooth. Because of this, many people choose to replace their old silver fillings with more durable composite resin.