Susie taught it to her. It was one of those little things that had never occurred to me... thinking about when peekaboo might turn into full-body hiding. (She's still a huge fan of peekaboo games.)
Tonight she hid around the corner from me. And after a moment, called out, "Mommy, come! Hide!" And held out her hand to me.
"But Calliope, if I hide with you, who will find us?"
"Mommy come hide."
So we hid together.
A moment later, she understood the limitations of hiding together, and scampered off to her room. Obediently, I started to count to ten... as she counted out loud along with me, from her "hiding" spot in her room.
"Ready or not, here I come!"
And then, too excited to wait, she came running out to find me.
With a saw.