Your two-year-old is more predictable to you now than she used to be. You know, for example, the difference between her behavior when she's overtired and when she's unhappy. In addition to noticing your child's atypical behavior, you should be aware of common childhood illnesses and their symptoms so you can assess the situation if your two-year-old should exhibit signs of illness. Having a more informed understanding of your child's medical condition can be helpful in supplementing your doctor's knowledge and care.