水烧开之前，为什么会咕噜咕噜响？

Listen to the water in the kettle. 听听水壶里的水。 Before it boils, it makes a funny noise. 还没开呢，它先咕噜咕噜响。 Small bubbles form at the bottom. 壶底先冒出一个个小气泡。 The bubbles rise, then pop in cool water. 气泡往上跑，碰到冷水又破掉。 That popping makes the gurgling sound. 破来破去，就发出了咕噜声。 When all the water is hot, it boils quietly. 等整壶水都烫了，反而安静了。 The loudest noise comes just before the boil. 最响的时候，就是水快开的那一刻。