The story in three lines. Lan and Inkling arrive in a storehouse in Babylon, where a boy learning to be a scribe must write down two hundred and five sacks of barley. A tiny gap for the empty tens place is read as twenty-five. A wide gap works until a scribe copies it in a hurry and squeezes it shut. Lan asks whether a mark could mean none. The boy presses two slanted wedges into the empty place, and the copy keeps it.

One question to ask. "Why did the big gap stop working?" Then: "What does the 0 in 106 do?"

One thing to do at the table. Write some numbers with a zero in the middle, like 105, 207 and 1,006. Read each one aloud. Then cover the zero with a finger and read it again.

The grown-up layer. The idea is a placeholder. A mark that means none lets a count show an empty place, so the other digits stay where they belong. Babylonian scribes used a mark like this by about 300 BC. It was not yet a number of its own, and the Honest Page says so. It also says they counted in sixties, not tens. This time Lan says the answer too soon and it buzzes. With this book the second word under DOOR is complete: OPEN.