# Day 78 — Swap A and B

**25 minutes · AC9MFM01 (capacity focus, partial) · educator review pending**

**Success:** Child checks the same unequal containers after their A/B labels are reversed and correctly names the new A as the greater usable capacity **because of the fresh transfer**, not because the earlier letter was greater.

**Prepare:** yesterday's unequal Pair B with labels physically swapped, now pretested and recorded as **Pair A**; C03/C05 child prompts and [R04/R05](print-aids.md), tray, water, towels, clear fill lines. Teacher checks the reverse transfer privately before class. Keep the reserve Pair F unused.

1. **Retrieve, 3 min.** Show the two dry containers with labels reversed. Ask, “Did changing their names change what they hold?” Elicit that a label alone cannot.
2. **Model, 5 min.** Read R04: B full empties into A with room left, so A holds more. Contrast it with R05 where B still has water when A reaches line, so B holds more. Say the **endpoint**, not the letter order, decides.
3. **Direct reverse test, 10 min.** Read C03. With new A at line and new B empty, adult transfers slowly under children's directions. In verified Pair A, B reaches its line while water remains in A. Children state A holds more to the lines. Repeat or discuss C05 with the same pair; teacher observes three focus learners.
4. **Fresh process check, 4 min.** Give the teacher-pretested Pair A with a new A/B card orientation. A focus learner states the start and stop conditions and directs another safe run. Record the actual outcome; the physical pair is familiar but the labels and direction were changed.
5. **Close, 3 min.** Compare yesterday's wording with today's. **Key:** Pair A / C03 / C05: A holds more, because B reached its line and A still contained water. R04 A more; R05 B more. If actual transfer contradicts the pretest, mark inconclusive and repeat.

**If a child says “B still has more because B won yesterday”:** Point to the physical container rather than its new letter, then ask what the new transfer showed. **Access and record:** Use attached tactile A/B labels, spoken endpoints, AAC and child-directed adult pour. Note whether a child switched label-reasoning to capacity-reasoning.