We were lolling about in a kiddie pool when my 19-month-old daughter, Claire, staggered out of the water and moved quickly toward the deep end of the big pool. Barely acquainted with the physics of solids and liquids, she didn't pause to consider the difference between the gray concrete walkway and the shimmering blue water. Lurching forward, she plunged. A half step behind, I jumped in, lifted her to the surface, and wondered: What on earth was she thinking?

I still don't know. The toddler brain is a magnificent specimen -- one day convincing its young owner that the shadows on the pool are fish, the next day sending the message that a 20-pound girl in a diaper can actually swim like one. Ideas are embraced and rejected at a dizzying rate; moods shift dramatically. It's like living with a very short, very opinionated Hollywood producer.

Toddlers are adorable, so we cut them a lot of slack. Still, all this flirting with staircases, hot toaster ovens, and the word "now" can be tough on both them and us. It's not that toddlers are trying to exasperate their parents; at this age, their abilities simply haven't kept pace with their ambitions. Between birth and 36 months, their brains crank out cells to develop memory, coordination, and self-control. Until it all falls into place, those skills and traits just aren't available on demand.

Understanding what toddlers don't know can give you a bit more patience the next time your toddler decides that as an artist, he simply must scribble on the kitchen cabinets. Here, the limits of a toddler's knowledge in four key areas:

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