小苏打为什么能让馒头变胖？

Mom puts a little soda in the dough. 妈妈和面时，会加一点小苏打。 The bun comes out fat and soft. 蒸出来的馒头，白白胖胖软乎乎。 Heat makes baking soda give off gas. 小苏打一受热，就放出好多气。 That gas is carbon dioxide. 这种气，叫二氧化碳。 Tiny bubbles get trapped inside the dough. 小气泡困在面团里，跑都跑不掉。 They puff the bun up like a balloon. 它们把馒头吹得像气球一样鼓。 Cool down, and the shape stays. 凉下来以后，形状就定住了。