Terrible Twos

The Terrible Twos is a child development stage where kids first realize they don't have to do everything their parents tell them, and they resist the simplest requests with disproportionate, violent responses. Thus asking a kid to give up a toy or brush his teeth can turn into a prolonged argument.

The person who invented the term has said that she regrets introducing it, because it gives two year olds a bad name. In actuality the behavior is commonly observed in one year olds as well, and can continue through adulthood and into old age for some individuals.