הוריות ב' ע"ב
אמר ליה אביי להוראה, גמיר וסביר משמע
Recipe for Knaidlach calls for four eggs
The maid came to the Bais Halevi on Yom Tov and said
"My Bal Habusta sent me to the Rov with this שאלה.
She told me to make Knaidlach using three eggs and I made it with four eggs. May we eat the Knaidlach?"
The Bais Halevi answered not to eat it in the meantime, but to come back again the following day with the same שאלה .
The onlookers found the question and the answer puzzling.
The Bais Halevi explained, The maid must have been nervous when asking the שאלה and left out part of the story.
The Bal Habusta probably told her to use three eggs because that's all the eggs they had in the house.The maid knowing therecipe calls for four eggs picked up another egg from the chicken coop.
.Egg was laid on Yom Tov (ביצה שנולדה ביו"ט)
He told her to come back the following day because the fourth egg is not בטל being it is a דבר שיש לו מתירין
After Yom Tov, it's not Muktza anymore therefore ביטול doesn't apply and one must wait to eat the knaidlach till after Yom Tov.
A Rav, not only needs to know the answer but also needs to know and understand the question ( שאלה )