Bothering Rav

שבת קח.

שמואל וקרנא הוו יתבי אגודא דנהר מלכא חזינהו למיא דקא דלו ועכירי א"ל שמואל לקרנא גברא רבה קאתי ממערבא וחייש במעיה וקא דלו מיא לאקבולי אפיה קמיה זיל תהי ליה אקנקניה אזל אשכחיה לרב א"ל מניין שאין כותבין תפילין אלא על גבי עור בהמה טהורה א"ל דכתיב (שמות יג) למען תהיה תורת ה' בפיך מן המותר בפיך מנין לדם שהוא אדום שנאמר (מלכים ב ג) ויראו מואב מנגד את המים אדומים כדם מניין למילה שבאותו מקום נאמר כאן ערלתו ונאמר להלן ערלתו מה להלן דבר שעושה פרי אף כאן דבר שעושה פרי אימא לבו דכתיב (דברים י) ומלתם את ערלת לבבכם אימא אזנו דכתיב (ירמיהו ו) הנה ערלה אזנם דנין ערלתו תמה מערלתו תמה ואין דנין ערלתו תמה מערלת שאינה תמה א"ל מאי שמך קרנא אמר ליה יהא רעוא דתיפוק ליה קרנא בעיניה לסוף עייליה שמואל לביתיה אוכליה נהמא דשערי וכסא דהרסנא ואשקייה שיכרא ולא אחוי ליה בית הכסא כי היכי דלישתלשל לייט רב ואמר מאן דמצערן לא לוקמוה ליה בני וכן הוה

Shabbat 108a

Shemuel and Karna were chilling on the bank of the river Malcha. They saw that the waters were rising and turning brown.
Shemuel said to Karna, "A great man is coming to town, and he has a stomach virus. The waters are rising in his honor. Go figure out what his deal is." So Karna looked into it, and met Rav, who had come to visit the town.
He started asking him questions to figure out what his deal is.
"How do we know that we can only use kosher animals for tefillin?" Karna asked Rav.
"We know it from the pasuk says, 'So that God's name will be present in your mouth.' Things that can go in your mouth are appropriate for writing God's name upon." Rav told him.
"How do we know that blood is red?" (Rashi explains he was asking about Nidda blood, which varies in color and Halacha.)
"The people of Moav saw the  blood and it had turned as red as blood."
"How do we know that berit mila is supposed to be done 'down there' and not somewhere else?"
"It says 'orla' by mila and it says 'orla' by fruit trees. Just like that kind of 'orla' produces fruit, so too, the 'orla' of mila is done in a place that produces offspring."
"Okay, but how do you know to draw a parallel from there? Maybe its talking about the 'orla' of the heart or the 'orla' of the ear? The word 'orla' is used in both of those contexts too!
"We prefer to draw a parallel from trees because the word 'orlato' is used in both contexts, as opposed to 'orlat' which is used in the context of the heart and the ear. Look buddy, what's your name?"
"My name is Karna."
"Karna? May it be His will that a horn (keren) sprouts between your eyes!"
Later, Shemuel invited Rav over for dinner. In an effort to cure him of his stomach virus, he fed him barley bread and fish pie (Soncino press translation), and lots of beer. He didn't let Rav use the bathroom, so the foods can properly cure his stomach virus and he would release it all healthily at the right time.
Rav got really annoyed that he wasn't being allowed to use the bathroom. He cursed and said, "Whoever is paining me like this should never have sons!"
His curse came true, and Shemuel never had any sons.
(As an apologetic measure for this curse, he would always let Shemuel enter the room first from then on. - Bava Kama 80b)