Why can't I say Kedushah by myself?
If you pray alone, you skip Kedushah. That is not a modern rule. It is in the Gemara (the Talmud).
In Berakhot 21b, Rav Adda bar Ahava (a rabbi of the Talmud) asks where we learn that one person does not say Kedushah. His answer is a verse: "I will be made holy among the children of Israel" (Vayikra 22:32). Anything that is a matter of holiness, he says, needs at least ten.
How does "among Israel" turn into ten? The Gemara links the word "among" (toch) to another verse, "Separate yourselves from among this congregation" (Bamidbar 16:21), and from the word "congregation" on to the ten spies. The Kaddish DeRabbanan prayer, earlier this morning, tells that part of the story.
The Gemara does not stop to say why a public is needed. It gives the verse. The Rambam (Maimonides, the great medieval legal scholar and philosopher) begins his laws of sanctifying God's name with the same verse, and there he is dealing with the hardest cases, where someone is forced, on pain of death, to break a commandment. Kedushah uses the same verse for something ordinary and daily.
The ten also answer together. The Mishnah Berurah says that if one person says "Kadosh" early, while the leader is still introducing it, some rule that he did not fulfill it, because the answer must be given by ten at once.
Who counts in the ten? The Shulchan Aruch says Kaddish, Kedushah and Barchu need ten adult men (Orach Chaim 55:1). The Mishnah (Megillah 4:3) lists other parts of the service that also need ten, such as reading the Torah and the Priestly Blessing.
Berakhot 21b,Megillah 23b,Rambam, Foundations of the Torah 5:1,Mishnah Berurah 125:3,Shulchan Aruch, Orach Chaim 55:1,Mishnah Megillah 4:3Isn't standing like an angel just pretending?
The prayer itself starts the comparison. The Ashkenazi wording of the opening says it plainly: "We will sanctify Your name in the world as they sanctify it in the highest heavens" (Tur). The Sefard wording says we sanctify God "like the sweet talk" of the Serafim.
Someone asked the practical question in the 1400s. Rabbi Yisrael Isserlein, a leading rabbi in Austria whose rulings are collected in Terumat HaDeshen, was asked whether an individual may put his feet together while saying Kedushah. He said yes. Since we say "as they sanctify it in heaven," and in heaven they sanctify with a straight leg, we should be like them as much as we can. In his words, this is not arrogance or foolishness, but a straight way of doing it.
The Shulchan Aruch only says it is good to keep your feet together. The Rema (the Polish rabbi who added the Ashkenazi customs to it) adds that people lift their eyes, sway, and rise off the ground.
Not everyone liked all of it. The Magen Avraham (a major commentary on the Shulchan Aruch from the 1600s) objected to people who skip and jump. Other later authorities defended the custom. The Mishnah Berurah's answer: a custom of Israel is Torah, but where it is not the local custom, it is obvious you should not start it, because people will end up mocking.
So the sources are open about it. It is meant as imitation. How far to take it depends on the custom of your community.
Tur, Orach Chaim 125,Terumat HaDeshen 28,Shulchan Aruch and Rema, Orach Chaim 125:2,Mishnah Berurah 125:8My friend's shul starts Kedushah with different words. Why?
Both wordings are old. The Tur (a law code from Spain, written in the 1300s) gives the one this siddur uses, Nakdishakh, and then adds: in Ashkenaz they say another version. He reports it and does not call either one wrong.
The other version begins Nekadesh et shimkha, "We will sanctify Your name." It is what many Ashkenazi siddurim use. Both versions lead into the same three verses.
This siddur follows the Arizal (Rabbi Yitzchak Luria of Tzfat, 1500s) here. The Kaf HaChaim (a large law commentary from the early 1900s) quotes his teaching as recorded in Sha'ar HaKavanot: in the repetition, say Nakdishakh, like the Sefardim, and not like the Ashkenazim, who say Nekadesh.
There is a detail in the word count. The Tur says the Ashkenazi lead-in has 14 words, "like the verse." The Kaf HaChaim, following the Bach (a major commentary on the Tur), uses the same count to fix the Sefard wording: with "as it is written" the lead-in is 14 words, but with "and so it is written" it would be 15. You can check it yourself. In the Hebrew, the words from Nakdishakh to "Your prophet" number 14, and the verse that follows, from "And one called" to "His glory," is also 14.
The Tur's only reason is that the count matches the verse. He does not say why that matters, and the sources quoted here do not either. What they show is that people counted the words of this prayer one by one and cared which version was said.
Tur, Orach Chaim 125,Kaf HaChaim on Shulchan Aruch, Orach Chaim 125:1,Sha'ar HaKavanot, quoted there