Measuring the effect of genes
Identical twins share identical genes, and are usually brought up in the same environment. Non-identical twins only share half their genes, but they too usually share the same environment. So something that is almost entirely under genetic control, for example eye colour, will be the same in all identical twins, but not in all non-identical twins. However, for something that has nothing to do with genes, like scars, there will be as many differences between identical as there are between non-identical twins.