As children grow closer to their second birthdays, they take big steps in learning about the desires and feelings of other people. They also become more assertive about their own wishes. Toddlers begin to understand that other people have desires and wishes different from their own, and this can lead to conflict. Since toddlers need to learn how conflict works between people, they begin a series of early experiments by testing boundaries and discovering their physical and developmental limits. Toddlers may test a limit set by an adult (by saying "no" or doing the opposite of what is asked) to see what reaction they will receive, even if they know the reaction will be negative. Although these defiant behaviors are healthy and developmentally appropriate ways for toddlers to deal with conflict, they challenge adults to develop responses that maintain necessary limits while firmly encouraging cooperation.