The Bee's Knees
A more obscure expression, "the bee's knees" means something is excellent. Magnificent. Top shelf. But do bees have knees, and if so, are they great? Like all insects, bees have six sections to their legs. Each segment connects to the next by a joint. One of the six sections could be considered more knee-like than the others, but in reality bees do not have knees in the way we think of them. Some posit that since bees have sacs on the back of their leg segments to carry pollen, "the bee's knees" was a reference to the fact that pollen was a good, or excellent, thing.
However, when the phrase became popular in 1920s America, there were other similarly silly animal expressions being bandied about, like "the cat's pajamas" and "the sardine's whiskers." The phrase likely developed simply as a sign of the times. And it couldn't hurt that "bees" and "knees" rhymed, making it fun to utter [source: McCabe].