8 to 12 Months
When your baby starts to crawl, around the 8-month mark, it's time to think about setting limits. Suddenly everything, from cords to toilet paper, is a big no-no. A child this age only wants to explore (she has no concept of what she should or shouldn't do), so if you don't want her to touch something, put it out of her reach and let safer items take center stage. This is the best way to keep her from doing something she shouldn't.
Of course, many of us merely say no when we catch our kids getting into mischief. Your child knows by the tone of your voice that "no" means something different from "I love you," but she doesn't understand the real meaning of the word. Use other techniques, such as distracting her, to reinforce the lesson that some things are off-limits.