One of the purposes of Mark Twain’s “Taming the Bicycle” is to entertain the reader. Which sentence in this excerpt uses humor to achieve this purpose?

We examined the machine, but it was not in the least injured. This was hardly believable. Yet the Expert assured me that it was true; in fact, the examination proved it. I was partly to realize, then, how admirably these things are constructed. We applied some Pond's Extract, and resumed. The Expert got on the OTHER side to shove up this time, but I dismounted on that side; so the result was as before. The machine was not hurt. We oiled ourselves up again, and resumed. This time the Expert took up a sheltered position behind, but somehow or other we landed on him again. He was full of surprised admiration; said it was abnormal. She was all right, not a scratch on her, not a timber started anywhere. I said it was wonderful, while we were greasing up, but he said that when I came to know these steel spider-webs I would realize that nothing but dynamite could cripple them. Then he limped out to position, and we resumed once more. This time the Expert took up the position of short-stop, and got a man to shove up behind. We got up a handsome speed, and presently traversed a brick, and I went out over the top of the tiller and landed, head down, on the instructor's back, and saw the machine fluttering in the air between me and the sun. It was well it came down on us, for that broke the fall, and it was not injured.