This grouping impulse is not only adorable, it's also an important developmental milestone, says Kyle Pruett, M.D., clinical professor of child psychiatry at Yale University. Between 18 and 30 months, toddlers begin to understand that objects, people, and animals all have different properties, Dr. Pruett says: "They realize that things can be hard or soft, blue or red, breakable or not breakable. This is the precursor for recognizing that the letter 'A' is different from the letter 'B.'" Kids also get a major sense of accomplishment from sorting. "The drive to classify objects is a very real itch they have to scratch," he says. "They'll sit back and scan their work with great pleasure."
Of course, the messy results -- stacks of Duplos, piles of stuffed animals -- can drive you bonkers. Best strategy: Be patient and enjoy watching your child's brain in action. "When kids learn a new task, they want to practice it over and over," says Dr. Pruett. "If you fight about it, it becomes a control issue, and it'll last that much longer."