You may remember this one from your babysitting days, or even your own toy box. And there's a good reason it's been around so long. "Rock a Stack" is perfectly geared to the mind of a baby who's around five months old, and is still fun for months afterwards. The concept is simple: five plastic rings in graduated sizes, each a primary or secondary color, that baby (or Mom or Dad) can stack on a plastic stake attached to a rocking base. Baby learns to recognize shapes and colors while improving her motor skills.