Reference for Kiddushin 159:6

ואין שורפין תרומה על החזקות דאמר ר' שמעון בן לקיש שורפין על החזקות ור' יוחנן אומר אין שורפין ואזדו לטעמייהו דתנן תינוק שנמצא בצד העיסה ובצק בידו ר"מ מטהר וחכמים מטמאין מפני שדרכו של תינוק לטפח

Now, they are in accord with their opinions. For we learnt: If a child is found at the side of a dough, and there is dough in his hand, R'Meir declares it clean; the Sages declare it unclean, because it is a child's nature to dabble.<span class="x" onmousemove="('comment',' Rashi: the child certainly took the piece from the dough, and since it is his nature to dabble among refuse and unclean things. he is probably unclean (which is regarded as a certainty) and so defiles the dough. Tosaf.: the child is certainly unclean (because women, even when menstruants, fondle children; Tosaf. Toh. III) and the only question is whether he took the dough himself or it was given him. The Rabbis declare the large dough unclean, because it is a child's nature to dabble with food, and so he probably took it himself.');">⁷</span> Now, we pondered thereon: What is R'Meir's reason?

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