Birds and Bees, if You Please

Millie and Susannah have both spoken some doozies lately (especially Susannah; she's an odd little duck), but since I didn't write them down, I've forgotten. Millie just delivered this 30 minutes ago, though, so it hasn't seeped out of my head.

While I was downstairs cleaning up after supper, I heard loud evidence that the girls were up to no good upstairs. I could track their actions simply by listening to the crashes and thumps. I didn't bother going upstairs to assess the destruction, but I told them they needed to clean up before our nightly Advent reading.

Ten minutes later, Millie appeared at the top of the stairs and heralded with a pompous boom, "COME! Come and see the SEA OF PROSPERITY!" And then again, with dramatic pauses, "THE SEA. OF. PROSPERITY!"

She ushered me into the bedroom, where Annie was perched with barely repressed excitement on top of their surprise-- John's and my bed, freshly made with hasty, little hands-- The Sea of Prosperity.

Apropos, considering our children are our wealth.

My question is this:
How on EARTH did she know?