Subtraction

Star, who has a naturally mathematical brain, is currently hopping around around in My Pals Are Here 3 (a newer version of Singapore Primary Maths). Yesterday we started subtraction with borrowing of tens and hundreds. Looking at the first column of units, I asked whether we could subtract 9 from 5, expecting the answer "no". "Yes," says Star, "it's minus four". My attempts at explanation dissolved in disarray. And no, I haven't taught her minus numbers. She has picked them up from the temperature gauge in the car.