The yeast comes alive and starts eating the sugars, creating carbon dioxide and alcohol — essentially it's burping and sweating, which is what bread is. It's yeast burps and sweat.
La levadura se activa y empieza a comer azúcares creando dióxido de carbono y alcohol, en esencia eructo y sudor que hacen al pan, es el eructo y el sudor de la levadura.