In the sixth month, your baby will continue exploring some of the basic laws of physics. Water spills, plates fall, and mom and dad make funny noises when something really messy hits the floor. Games of "cause and effect" may not be good for your carpeting, but they work wonders for her brain. She's learning that she has some control over the world, and she's beginning to understand that actions have predictable results.
Playtime is an especially good time to fuel her curiosity. You don't have to buy her a lot of fancy toys, either. At this age, she'll love to play with boxes, spoons, cups, and plastic containers with lids. A soft, pliable ball is a great device for testing gravity. She'll also love to play copycat. Shake a rattle or push a button on a toy, then give her a chance to follow your lead.
She's still a long way from saying her first word, but her brain is already trying to crack the code of language. She'll start to make distinct sounds, including long vowels and strings of nonsense syllables. You can encourage her language development by talking to her often. Name things that she has in her hands, such as a toy, book, or ball. Point out things you see on your walks, like a tree, bird, or dog. Use her name when talking to her. And when you hear her make a recognizable syllable, show her how that sound can be part of a word. If she says "mah," you can respond with "mama" or "mother" or even "moth."
She may be discovering another aspect of language: The art of yelling. When she starts screaming recreationally, you should respond in a whisper. She'll quickly quiet down. For her, it's just another fun game of copycat. For you, it's some much-needed relief.
American Pregnancy Association. First year of development. http://www.americanpregnancy.org/firstyearoflife/firstyeardevelopment.html
Sears, William and Martha. The Baby Book: Everything You Need to Know About Your Baby. From Birth to Age Two. 2003. Little, Brown and Co.
American Academy of Pediatrics. Caring for Your Baby and Young Child: Birth to Age Five. 2009. Bantam Books.
American Academy of Pediatrics, HealthyChildren.org. Cognitive Development: 4 to 7 Months. http://www.healthychildren.org/English/ages-stages/baby/pages/Cognitive-Development-4-to-7-Months.aspx