In Gertrude McFuzz, Gertrude's tail is described as being as gorgeous as diamonds, gumdrops, gold, silk, or spaghetti. This humourous analogy is a subtle foreshadowing to the aesop that having beautiful accessories doesn't make you a beautiful person.
Some people have read The Zax and wondered why one of them didn't just leap-frog over the other. Well, given their nature, it's likely that they would get into an argument as to who should jump over who. Besides, there's the possibility that up and down are considered different directions than north or south.
Green Eggs and Ham teaches a lesson not only to the unnamed character, but to Sam as well. He only managed to get the unnamed character to try the eponymous dish when he asked nicely instead of trying to force him.
The Lorax teaches a lesson not only to the Once-ler, but to the title character. If he had been a little less preachy, maybe the Once-ler would have listened. The moral? Don't be too preachy.
Also from The Lorax: why do we only see the Once-ler's arms? Well, the boy in the framing story only sees his arms, and is referred to as "you", meaning he represents the reader. The illustrations of the main story are what the boy is imagining when the Once-ler tells his story.
The Once-ler's gloves are green, a colour symbolizing both greed and environmentalism. What could be a more perfect colour to symbolize his Character Development?
Thidwick the Big-Hearted Moose and Horton Hears a Who seem to teach opposite lessons about kindness; while the former explores the necessity of limiting your kindness, the latter explores a situation where kindness must be unlimited. Well, Thidwick's and Horton's situations were completely different, and when read together, they give a complex message about kindness. Horton wasn't exactly "kind" to the kangaroos and Wickersham brothers, either.
In The 500 Hats of Bartholomew Cubbins, Wilfred keeps suggesting that Bartholomew be sentenced to death and even attempts to push him off the turret when the King tells him to wait. Why does he hate Bartholomew so much? Well, when he was about to shoot off Bartholomew's hat with his child's bow, Bartholomew proudly told him that he could shoot with his father's bow. Wilfred was probably just jealous that Bartholomew could shoot with a bigger bow than he could, and the fancy new hat Bartholomew was wearing on top of the turret made him even more jealous.