Reference for Kiddushin 160:2

א"ר יהושע בן לוי לא שנו אלא במשקים לבנים אבל במשקים אדומים אם איתא דנקיר מידע ידיע ודילמא בלעתינהו עיסה

it would certainly be known. Yet perhaps the dough absorbed it? - Said R'Johanan: Beribbi<span class="x" onmousemove="('comment',' V. p. 101, n. 8. Here referring to R. Joshua b. Levi.');">⁴</span> heard this thing, but not its explanation [which is this]: We learned this only of clear flui in which a child's reflection may be seen but not of turbid fluid.<span class="x" onmousemove="('comment',' Both refer to coloured liquid. If clear, it soaks in easily, and the dough is therefore unclean. Turbid liquid, however, must leave some traces; hence it is clean.');">⁵</span>

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