Answer:
the baker accidentally left the grain paste in the stand and it collects wild yeast from air.
Explanation:
The grain paste left to stand for a time sooner or later collects wild yeasts form air, and begins to ferment. This was the beginning of leavened bread, although for most of human history the presence of yeast was mostly accidental.
It is believed that the use of sourdough in bread leavening production developed in ancient Egypt in approximately 3000 BC and from there spread gradually to Europe, throughout ancient Greece and the Roam Empire until the present.
