| Quote #4
"Magic takes just as much energy as if you used your arms and back. That is why you felt tired after destroying the Urgals." (19.65)
You wouldn't think that a supernatural skill would take such a physical toll on a person. In other words, Eragon can't simply wave a wand around, a la Harry Potter. His power comes directly from his own physical being. It takes something out of him every time he uses it. In that way, his magical powers stem directly from his personal strength and stamina.
| Quote #5
The clashes lasted longer as he learned how to fend off Brom. Now, when they went to sleep, Eragon was not the only one with bruises. (20.73)
Yes, young grasshopper! You have grown stronger! Oh, sorry, we were having a Kung Fu flashback. Much like Caine in that old TV series, Eragon is gaining strength thanks to his master's instruction. Soon, he will "snatch the pebble" from Brom's hand. This makes a lot more sense if you watch the show (which you should—trust us).
| Quote #6
Brom tossed what remained of his stick into the fire and said, "We're done with these; throw yours in as well. You have learned well, but we have gone as far as we can with the branches. […] It is time for you to use the blade." (21.70)
Oh, snap. Blade? Who said anything about a blade? Here Eragon has graduated from whack-a-stick to full-on, blade vs. blade action. Brom is upping the ante as Eragon gets stronger. It seems that, as he makes progress, the bar continues to shift higher and higher for him. Of course, how else would he improve?