Watch Where You Look

Before receiving the Torah, there were preparatory days wherein the Jews got themselves ready for the holy day. Hashem instructed Moshe to tell the Jews not to ascend the mountain; a boundary was set up which the Jews were not to cross. They were severely warned not to do this. The second time Moshe was preparing to receive the luchos, Hashem gave similar instructions, adding that no person should be present when he was to ascend, and even animal should not graze there.
From Hashem's instructions that upon ascending the mountain the animals should not be around, the Rokeach writes that this hints to the following idea: On the day we initiate a child to learning from the holy Torah, we cover him so he shouldn't see any dogs.
רוקח הל' שבועות ריש סי' רצ"ו
ועי' בלקו"ש חכ"ה ע' 310
See here

Using a Magnifying Glass for Halachah

The Aruch Hashulchan writes:
"There are those that say in the name of scientists that if one looks through a magnifying glass, they will see many insects in vinegar. Now, with vinegar, there is no problem... but I hear that all water, specifically rain water, is full of microscopic insects that are not visible to the naked eye. In my youth, I heard from someone that traveled far and used a microscope that enlarged the object thousands of times, and he saw all these insects. If so, how can we drink this water?
The truth is that the Torah does not forbid that which the naked eye cannot see, for the Torah was not given to angels. If one does not say so, now are there not scholars who say that the air is also filled with microscopic insects and when a person opens his mouth, he swallows many of them?! Surely they are speaking nonsense! Even if it is indeed so, being that the naked eye doesn't see them, it is nothing..."
He says the same thing about microscopic scales on fish - it still doesn't make them kosher:
"...Know, it is given, that with anything that needs to be visible, seeing it through a magnifying glass does not qualify; the naked eye must be able to see it. This holds true with anything in the entire Torah that needs to be visible."
ערוך השולחן יו"ד סי' פ"ד סל"ו וסי' פ"ג סט"ו
ועי' חכמת אדם כלל ל"ח בבינת אדם סקמ"ט
ועי' טוב טעם ודעת (למו"ר שלמה קלוגער) מה"ת קו"א (הל' טריפות) תשובה נ"ג
See here, and here

Righteous Women

The Kiyor in the Mishkan was fashioned from what the Torah calls "ma'aros hatzovos - mirrors of the women who had set up the legions."
Rashi explains that these mirrors were brought to Moshe by the women of the time. Moshe was disturbed with this donation, as they were used in Egypt to entice their husbands to have children. Hashem instructed him that this was indeed a virtuous act and most dear to Him; Moshe accepted them and the Kiyor was constructed.
R. Avraham Ibn Ezra takes a different approach:
"The way of all women is to beautify themselves by looking at their faces each morning with a copper or glass mirror and fixing the hair style on their head. This is referred to in Sefer Yeshayah, for the custom of Israel is as the custom of Ishmael until today. There were righteous women is Israel, servants of G-d who cast away the lures and pleasures of this world and donated their mirrors, for they did not need to beautify themselves. Rather, they would come every day to the doorway of the Mishkan to pray and hear the words of G-d's commandments."
פי' ראב"ע שמות לח, ח
ועי' רמב"ן שם ובתולדות יצחק שם (ועי' בסיום דבריו שהספור במשכן וכליו וצרותו אע"פ שאין בית המקדש קיים מצוה גדולה ושכרה גדול)

Help Her Out

Just as a man is supposed to help his wife with lighting Shabbos candles by preparing the candles, etc. he is supposed to ensure that the lights in all the other rooms are lit. The woman acts as his shliach when she lights, and as such, he is responsible for the other rooms that she doesn't take care of. So, erev Shabbos, the man goes around the house and makes sure that all lights that need to be on are on. However, a brachah is not made; his wife's brachah takes care of that.
שו"ע הרב סי' רס"ג ס"ה, ס"ט
ועי' אליה רבה סי' רס"ג סקט"ו ובמה שנסמן שם
וע' קו"א בשו"ע הרב שם סק"ב

Active Martyrdom

The Beis Yosef relates the following story:
"Once a rabbi slaughtered many children during shmad, because he was afraid the enemy would make them leave their religion. Another rabbi present was angry at him and called him a murderer, but he did not listen. The opposing rabbi declared, “If I am correct, may that rabbi be killed in an abnormal way!” And so it happened; the non-Jews caught him, flayed his skin and placed sand between the flesh and the skin. Afterwards, the decree was nullified, and had he not slaughtered them, they may have possibly been saved and not killed."
ב"י (בבדק הבית) יו"ד סי' קנ,ז ד"ה ומ"ש רבינו על שאר עבירות, ע"ש אריכות השו"ט בזה

Don't Rule Without Him

R. Chaim Benveniste was a prominent rabbinic authority in 17th century Turkey. A student of the Mahari"t (R. Yosef of Truni) in Constantinople, he served as the chief Rabbi in the city of Izmir, Turkey. He authored many books of halachah, the most well known being the "Kneses Hagedolah" on Shulchan Aruch - further halachic decisions culled from numerous sources preceding him.
So important is this work, that the Chid"a writes: "...He who is well versed in the ways of deciding halachah and understands the gravity thereof should not decide any matter prior to consulting with this work. He should run to the study hall and see what he (R. Benveniste) says and what his wise eyes did indicate. Afterwards, upon leaving the study hall, he shall calmly give instruction..."
שם הגדולים מערכת גדולים אות ח
See here

Live Body, Lighter Body; Dead Body, Heavier Body

Did you ever wonder why a dead body is heavier than a live body?
When R. Yochanan Ben Zakai uncovered his plan to sneak out of the besieged walls of the city of Yerushalayim to visit the General Vespasian by faking as dead in a coffin, he instructed his students that only they shall carry his coffin. The Gemara (גיטין נו, א) explains that he knew that a dead body weighs more than a live one, and thus, he did not want anyone to uncover his disguise by detecting the unusually light coffin.
But why is this indeed so - what makes the body lighter while alive?
R. Bechaye explains that the living person is comprised of a material body and a spiritual soul. The soul enlivens the body and "carries" it, thus making it lighter. When the soul departs upon the person's death, the body is left in its heavy material form with nothing carrying it. This is the true meaning of what the Gemara (שבת צד, א) says "החי נושא את עצמו - a living body carries itself."
The same can be found with inanimate objects as well: When one takes molten iron directly from the fire, it is lighter than when after it cools off. This is because immediately after emerging from the fire, the metal still has some of the fire - the soul - in it; this "soul" carries the metal, thus making it lighter.
רבינו בחיי עה"ת שמות לב, טז
ועי' "המעין" שנת תשד"מ גליון תשרי ע' 55 מאמרו של חיים סבתו בענין חי נושא את עצמו, ותשובתו בצידו ב"המעין" שנת תשד"מ גליון ניסן ע' 63
See here