英语时差：枫糖浆到底是什么？（下）

And any year with extreme variations in temperature from night to day tends to be a good year for maple syrup as well. All trees produce some natural sugars, which they use for energy in their own growth processes, but not all trees, or even all maples produce enough or the right kind of sugar to make syrup. Out of about 100 species of maple trees, only four are good for syrup and of those, one species — the rock, or hard, maple — produces most of the syrup we use. If you’re wondering why something that flows from trees should be so expensive, consider how little syrup you get when the sap is boiled down. The pint jug of syrup you buy in the store came originally from about five gallons of sap.