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Because it's Saruman's palantir. The fact that a hobbit “answers” it leads Sauron to assume that Saruman has managed to capture the halfling that has the ring, and is using the palantir to torture the hobbit for LOLs.
In The Lord of the Rings: The Two Towers, there is a scene in Osgiliath where Frodo quite clearly offers the ring out to one of the Nazgul. In The Lord of the Rings: The Return of the King, Pippin touches the palantir, and it is established that Sauron now thinks Pippin has the ring thereafter.
This was a challenge to the Dark Lord, and when Aragorn dared to march on the Black Gate, Sauron was convinced that the Lord of Gondor was wielding the One Ring. The only logical course of action for Sauron was to empty Mordor in a last-ditch effort to get his ring back.
Galadriel suggests that Sauron never became definitively aware that she bore Nenya: Verily it is in the land of Lórien upon the finger of Galadriel that one of the Three remains. This is Nenya, the Ring of Adamant, and I am its keeper. '[Sauron] suspects, but he does not know - not yet.
She already feels an outcast after Gil-galad tried to send her back to the Undying Lands, and she fears that if she reveals what she has done, it will be the final nail in the coffin. Instead, when Elrond pulls her from the river Glanduin, and she races back inside to Celebrimbor's forge, she chooses to stay silent.
Though Galadriel doesn't share her news, Elrond is suspicious and wanders out to find the genealogy scroll that leads him to figure out that Halbrand is Sauron. Unfortunately, he finds this news too late, arriving back at the workshop just as the elven rings (the rings of power) are being completed.
As for Sam, his resistance likely has to do with his innate goodness and loyalty. His love for Frodo helped him to overcome the pull of the Ring and his inner pureness was never fully pierced by the Ring.
The Three Rings of the Elves were forged by Celebrimbor alone, and were never touched by Sauron. They were made last, however, and their forging involved some of the arts taught by "Annatar".
Sauron did not assist in their making nor ever did he touch them, and his taint was not directly upon them. But, as they were created according to the craft taught by him, they would fall under the power of The One Ring.
So why didn't he do anything about it? When Frodo finally arrived at the lands of Mordor, Sauron likely knew about his presence. He was well aware that someone was traveling with his One Ring and also that a Hobbit was found creeping through his home, so it would seem obvious to take this as a major concern.
Tolkien explains this in the book when Aragorn describes the Nazgûl to the Hobbits. They see things which our eyes do not see, but they are also blind to things our eyes do see. When Frodo puts on the ring on Amon Hen, he makes himself visible to Sauron within the unseen world.
Tolkien suggested that Sauron did not discover where the Three were hidden, though he guessed that they were given to Gil-galad and Galadriel.
In an interview with WIRED, Tolkien expert Cory Olsen answered this question. Olsen said that the reason Sauron does not turn invisible when he puts on the Ring is because, quite simply, he does not have a body.
Sauron probably did not view the Hobbits, if he did indeed know of their existence, as important enough to try to control. Most Hobbits do not have lofty aspirations, only wanting to live a peaceful life. Peace, of course, is no use to Sauron. He wants to dominate and control all of Middle Earth.
His age and experience are directly opposed to Pippin's youth and ignorance. For a very long time, Gandalf does not seem to approve of Pippin. He is mean to Pippin though he is only sparingly gruff to the others.
While the previous 19 rings were crafted with the help of the Elves, the final ring, the 'One Ring to rule them all,' was crafted by Sauron alone; with the ability to dominate the other rings, the One Ring was made with some of Sauron's own power, in order to make it more powerful.
This is really a selfless act because if she accepted the ring she would have the power she desires but many people would be living in fear under her power too. Really, Galadriel would probably make a good leader of Middle Earth on her own, without the ring.
About 500 years into the Second Age, Sauron reappeared, intent on taking over Middle-earth and ruling it as a God-King. To seduce the Elves into his service, Sauron assumed a fair appearance as Annatar, "Lord of Gifts", befriended the Elven-smiths of Eregion, led by Celebrimbor, and counselled them in arts and magic.
It's a constant reminder to Aragorn of his family history; his hesitation to take the ring from Frodo Baggins (Elijah Wood) is a direct result of Isildur's corruption. Although Boromir ultimately falls victim to the seductive nature of the ring, Aragorn is able to defy his family history and resist his urges.
No, Bombadil would not be a good Ring-bearer for the Quest. This is thoroughly discussed at the Council of Elrond. One of the elves says tha... Tom has much more important quests that destroying the One Ring, such as carrying the last lilies of the fall home for his wife Goldberry.
Sam does take the Ring for the right reason, to keep it out of orc hands at Cirith Ungol, but again his own weakness tempts him to keep and wield the Ring — even to challenge the Dark Lord.
It is therefore unlikely that Galadriel would have been able to save him or turn him towards the light if she had agreed to rule beside him. She even tells him this herself when she whispers: “No penance could erase the evil you have done.”
The Dark Lord easily overpowers her, and after Galadriel rejects his proposal to join him, he traps her in illusions and leaves her to drown in the water.
But when it becomes clear to Galadriel that Halbrand/Sauron cannot tell the difference between “saving” and “ruling” Middle-earth, she rejects his proposal.