Why did he ask Sancho for the magic bowl?
During a fight with a barber, Don Quixote steals a bowl from him. He is convinced that the bowl is the magic helmet of Mambrino. In the ensuing melee, Sancho Panza, the Don's faithful retainer, takes the opportunity to steal the barber's packsaddle.
Later on, when the three men meet up again, the barber, in front of a group of travelers, accuses the Don and Sancho of theft. Quixote is defiant. He loudly declares the barber to be under the sway of the magic helmet's enchantment. As far as the Don is concerned, he won the "helmet" fairly in righteous combat, so he's not about to give it back. Although it may seem strange that a humble barber's bowl could turn into a magic helmet, this is the kind of transformation that the Don claims is wrought by chivalry.
To prove his point, Quixote orders Sancho to fetch the bowl. Sancho doesn't think this is a very good idea, but the Don insists so he gets it. When Sancho comes back, the Don proudly lifts the bowl up in the air. He proclaims it to be the magic helmet that he took from the barber after their fight. Once again, we can see the stark difference between the real world and the chivalrous fantasy world Don Quixote inhabits.
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