4 to 6 months
What Your Baby Learns: She's figuring out that her actions cause predictable responses: When she shakes a rattle, it makes a noise; when she smiles, you smile back. She's also developing the ability to take in information received from two senses at the same time (such as seeing you talk to her as she listens to your voice).
Brain Boosters: Give her a chance to make things happen. She'll be fascinated by small items she can grab -- plastic keys, soft balls with bells inside them. She also likes listening to music as she plays and will gravitate toward toys that make noise and light up.
Keep in Mind: Make sure visits from friends and family are short: Four- to 6-month-olds enjoy some social interaction, but they can get overstimulated by too much of it. You should also try to maintain a consistent daily routine to prevent fussiness.