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Source and decoding
אמר רבא: עכבר נכנס וככר בפיו, ונכנס אחריו ומצא פירורין — צריך בדיקה, מפני שאין דרכו של עכבר לפרר
Rava said: A mouse entered with a loaf in its mouth, and one entered after it and found crumbs – a search is needed, because it is not the mouse's way to crumble.
Rava distinguishes between a mouse and a child. If a mouse entered with a loaf and crumbs were found, the crumbs are not assumed to come from its loaf, so a search is needed. With a child, crumbs found need no search, because a child's way is to crumble. This rests on the typical behavior of each, not on a general rule.