[He hesitates; he's not sure if it's okay to tell people about the Once-ler's story. In fact, he's pretty sure the Once-ler might have only told him because Ted was the first who bothered to ask, and then refused to leave]

Well. The trees were all cut down and there was a lot of smogulous smog and glopulous glop. The animals couldn't live there anymore, so they left.