A clay ball is rolled into a sausage. A child says, 'You could roll it back, so it is the same.' This shows:

1. The child justifies conservation by imagining the action undone. 2. The ability to think through the steps of a process in reverse is **reversibility**. 3. Piaget saw reversibility, with decentration, as what lets concrete operational children conserve. 4. This particular reason (roll it back) names reversibility directly. **Why not the others:** Decentration would sound like 'it is longer but thinner', attending to two features at once. _Source: Seifert and Sutton, Educational Psychology, 2nd ed. (2009, CC BY 3.0), Ch 3, section "The concrete operational stage: age 7 to 11", p. 49_