Dentists and researchers have come up with the formula for the perfect smile，from the size and width of each tooth to the curve，colour and shape of the teeth，and the overall width of the smile.
　　Those pursuing perfection are advised to hide their gums，hold back on the bleach，cover the bottom teeth and set their lips at just the right angle."Everyone wants the perfect smile and Jessica Simpson is the one we get asked to reproduce the most，" says Dr Nicholas Davis of Loma Linda University School of Dentistry.
　　According to the report，colour is important，and too many are now bleached too white. Dr Davis said："The teeth should be the same colour as the whites of your eyes. If the teeth are too white，they become more dominant than the eyes."There are strict mathematical ratios governing the ideal smile，says Dr Davis. Its width should be no less than half the of the face at the same height and，for the best effect，both upper and lower lips need to be symmetrical each side of the mid-line of the face.