Triangle and Square are visiting Circle, who lives at the waterfall. When they play hide-and-seek, Circle tells the friends the one rule: not to go behind the falling water. But after she closes her eyes to count to ten, of course that's exactly where Triangle goes. Will Circle find Triangle? And what OTHER shapes might be lurking back there?

Mac Barnett is the author of several picture books, including Chloe and the Lion and Guess Again!, which TIME Magazine named the no. 2 Picture Book of 2010. He also writes the Brixton Brothers series of mysteries. Mac is the founder of the Echo Park Time Travel Mart, a convenience store for time travellers, and serves on the board of 826LA, a not-for-profit writing and tutoring centre in LA. His collaborations with Jon Klassen, Extra Yarn and Sam & Dave Dig a Hole, have both won the Caldecott Honor. He lives in California, USA.