Where does the sneak come from?

I gotta just ask, how on earth do kids learn to be so sneaky so young? I know they don't just learn from each other, because Andrea was a super-stealth before Lacie came along. And I'm not talking the run-of-the-mill "it wasn't me" with her hands covered in peanut butter scenario. No, no. I'm talking about a full-on 'realizes as she's reaching for something obviously forbidden that I'm watching so she quickly changes direction to "pet" the lampshade and say, "Pretty".'
Seriously. Where does a kid learn a thing like that? Or like when I found her cradling what I thought was a doll, only to find out that she'd emptied a bag of marshmallows into a blanket and faked that it was a baby while she slyly slipped the marshmallows out when I wasn't looking. Ummm...? Hello?
And it's not just Andrea either. Lacie is just as good, if not better. It started with petty theft - casually standing by, watching Andrea playing with toys only to swoop in at just the right time, swipe a toy, and take off running. Then she learned evasive manuvers like shoving the toy under the couch when Andrea was hot on her trail. Next she just perfected her art of pick-pocketing by using distraction techniques. Now she is doing the full-blown manipulation. I mean, what am I supposed to think when my kid comes running up to hug me, then just lays there in my lap while I hold her -- and then I find out she's doing it because she swiped some other kid's pacifier and is secretly sucking on it with her head buried in my lap so that I can't see! Honestly!