"The first day of school, the teacher wants to know how well her new class is doing in maths.
She asks Johnny “If you had 20 goats in a field, and one found a hole in the fence and got out, how many would be left?”
“None” replied Johnny.
“OK, so you don’t understand subtraction yet. There would be 19 goats left.”
“Miss I do understand subtraction. You just don’t understand goats.”