The story in three lines. Lan and Inkling arrive at a fishing camp on a great lake in central Africa, about twenty thousand years ago. A girl there must bring in every fish trap before the camp moves tomorrow. Counting on her fingers fails, and her grandmother's pebbles half work until they spill. Lan asks whether the marks could be cut into something. Then she asks whether they could be cut in groups, one for each place along the shore. The last uncrossed notch shows exactly where the twelfth trap is.

One question to ask. "Why did the notches work when the pebbles did not?" Then: "What did the groups tell them that a long row did not?"

One thing to do at the table. Count something you can't keep in your head, like the cars that pass in five minutes or the birds in the garden. Make one mark for each, then try it again with the marks in groups.

The grown-up layer. The idea is one-to-one matching. One mark stands for one thing, kept outside your head where it cannot be forgotten, and grouped so it can be read. This time Inkling names the answer too soon and it buzzes, so the story is also about looking before naming. The real notched bones from Ishango and Lebombo are on the Honest Page, with the truthful note that nobody knows what their notches were for.