Well-known short stories from the Golden Age of science fiction by Lewis Padgett (actually a pen name for Henry Kuttner's collaborations with his wife, Catherine L. Moore). First published in Astounding Science Fiction in 1943, it's since been reprinted in various collections, and you can probably find it online. The story is funny and engaging, so I've tried not to give away too much, but there might be minor spoilers.

The premise: millions of years in the future, a scientist belonging to a post-human race builds two time machines. At the last minute, he realizes he needs test matter to go inside them, so he fills them with the nearest things at hand: his grown son's discarded toys. However, the experiment is a failure, as neither time machine returns to his present.

Cut to our present. The year is 1942, and a five-year-old, Scott, finds the second time machine. The toys inside give Scott and his sister, Emma, a basic post-human education. This includes non-Euclidean geometry and the seemingly random "X logic". Since they are children (and thus not yet conditioned to traditional logic and Euclidean geometry), they are able to understand all this, while the adults can see only nonsense. The younger Emma takes to X logic faster than Scott, and seems to be helping him with something. Perhaps instructions left by the finder of the first time machine?

The "serious" side of the story is a philosophical debate concerning the nature of humanity. According to the theory of one of the characters, because a baby thinks so differently from an adult human, it isn't quite human. Neither sub- or superhuman, just different, understanding things adults can't and not understanding things adults find easy. Interestingly enough, the same is said about the differently thinking post-humans who made the toys.

The title is a line from Lewis Carroll's poem Jabberwocky. As the ending reveals, it is not a nonsense phrase.