1. The problem states that the length $l$ of a pencil is 12 cm to the nearest centimetre. 2. When a measurement is rounded to the nearest whole number, the actual value lies within half a unit below and half a unit above the rounded number. 3. Since $l$ is rounded to 12 cm, the lower bound of the error interval is $12 - 0.5 = 11.5$ cm. 4. The upper bound of the error interval is less than $12 + 0.5 = 12.5$ cm. 5. Therefore, the complete error interval is: $$11.5 \leq l < 12.5$$ 6. The number that should go in the box is $12.5$ cm.