How can changes in moles indicate skin cancer?

So I always tell my patients to look out for the ugly duckling. So this is the thing that doesn't look like the other moles or lesions that you have. A very helpful mnemonic to remember is A, B, C, D, E. A for asymmetry, B for irregular border, C for colour variation, D for increasing diameter and E for evolution. The E probably being the most important, so any change that you notice should be checked.