It's a general rule that babies can't talk. That's part of the deal: you get a wee baby with no real skills whatsoever except to eat and sleep. Pooping, even, is a learned skill that one must acquire out of the womb.
But of course, mothers & fathers have been able to understand their babies' wants one way or another - some listen for certain frequencies in the baby's cry, others look for gestures, and there are those enterprising parents who wish to teach their children language as early as possible. We take more of a laissez-faire approach - maybe that's the beauty of the second child - but we can still determine to some extent the needs of wee Elsie.
How, you may ask? By a quick glance at her toes.
She is a very physically-centered baby (most babies are, I suppose, but Elsie seems to be moreso than Olivia was), and loves to flail her appendages about. When she sees something that she likes or wants, particularly at this point something she wants to chomp on, her body flexes - her fists fist up more (I would say hands, but they're generally fists in the first place), her eyes get big, her legs straighten out and her toes - therein lies the secret - her toes stretch out as far as they everly can.
We call it toe-rometer, although that is a rather clumsy name. If you've got a better one, let me know.
EXCITEMENT: Toes and their accompanying feet move about with great alactrity. Toes are stretched out, and
TIRED EXCITEMENT: Same as "excitement" but accompanied by crying.
RELAXED: Toes are relaxed and wiggly. Sometimes toes are in mouth.
BUSY WITH PLUMBING: Toes are curled up, legs extended. Usually accompanied by grunts.
Of course, this doesn't begin the cover the gamut of a baby's emotions, but it's certainly a beginning. Look for a forthcoming dictionary. Or not.