Asked how far apart -6 degrees Celsius and 9 degrees Celsius are, a learner subtracts 6 from 9 and answers 3 degrees. How many degrees apart are the two readings?

Erklärung

Counting from six below zero up to zero uses six degrees, and climbing on to nine takes nine more, so the two readings stand fifteen degrees apart. Subtracting the sizes assumes that both readings sit on the same side of zero, which is exactly what the minus sign rules out, while counting every whole-degree mark from one reading to the other, both ends included, overshoots the gap by one.