Sneezing from sawdust, the Lorax introduces himself as the Lorax, he who speaks for the trees (because trees can't talk).
Just to recap: trees can't talk but they have a little talking guy inside them. Yeah.
Short of breath and standing on his stump, the Once-ler hurls his one question at the green-gloved Thneed maker (whose face is never shown, by the way):
"What's that THING you've made out of my Truffula tuft?" (100).
(Ah, the power of italics! This might also be a good time to explain to your future texter/e-mailer that all caps means YELLING.)
The Once-ler isn't fazed by the blustering Lorax. He keeps it cool and tries to convince the Lorax that (a) it was just one tree, (b) he's not doing any harm, and (c) Thneeds are good and needed in the world.
Curious about this mysterious Thneed? "It's a shirt. It's a sock. It's a glove. It's a hat" (105). Still not convinced? Well, "You can use it for carpets. For pillows! For sheets!" (107).