reschoolers have a keen sense of their own power and practically no notion of danger. This means they perceive no limits on what they can do and have no cognitive ability to understand that many things are inherently unsafe. A two-year-old will avoid a hot stove or an open window only because his parents tell him to, not because he senses the risk.
Young children learn about the world, including what is safe and what is not, through firsthand experience. And the learning process is gradual. This is why parents have the tedious job of saying no over and over in the course of a day. Why do children so often ignore their parents' warnings? A child touches the stove even though you say "Don't," because that's the only way he'll learn what hot means. What usually happens is that he feels the intense heat before actually touching the burner, quickly withdraws his hand, and never touches the stove again. Now he understands this danger and will avoid it in the future.
At this stage in their development, children are moving away from dependence on their parents toward greater personal independence. They are constantly pushing themselves and their parents to the edge in order to test the limits of what they can do. Preschoolers must feel free to explore the world without too much restraint.  The parents' job is to provide what I think of as a trustworthy environment, in which their child can take certain risks without paying too high a price.
Inevitably, blistered thumbs, skinned knees, and bruised elbows will accompany a child's risk taking. This is a source of anxiety for parents; however, we can't be with our children every minute to protect them against all possible harm. And even if we could, we would not be doing them a favor; they would never learn to protect themselves from the dangers they are likely to face as adolescents and adults.
Children acquire a self-protective mechanism over time as they learn through experience and as their thinking process matures. Parents sometimes believe that child-proofing and vigilance are no longer necessary once a child reaches the age of three, four, or five and can competently negotiate her environment.
This assumption almost always turns out to be wrong. A four-year-old now knows more than she did when she was two, but her cognitive faculties are not yet developed enough for her to grasp the dangers in every situation. She may think she can reach a book on a high shelf, but won't know that pulling it out may bring a whole stack of books down on her head. Each new experience helps her learn about the world, but her ability to judge potential danger is still rudimentary.
The parents' most important role is to help their child strike a balance between risk taking and a feeling of security, between experimentation and caution, between independence and dependence. Certainly, this is difficult to manage. Bear in mind that safety is one of those issues in which parents' actions and attitudes may count as much as or even more than their words. If you are overly anxious, for instance, you may make it harder for your child to become a confident and curious individual. (Sometimes he will react to a parent's  excessive worry by becoming extremely adventurous, even reckless.)
Even as children are taking risks and pushing limits, they are learning by what they see their parents do. If you don't observe the basic rules of safety (wearing a seat belt, using a pot holder to remove something from the stove), your children are bound to be confused when you tell them about the importance of avoiding unnecessary risk. At the same time, if they see you buckle up that seat belt and in other ways practice what you preach, providing a trustworthy environment will be that much easier.