What tells the reader that the speaker of the poem " Going Down Hill on a Bicycle A Boy's Song knows his joyful experience is temporary?

Which two words are nonsense words in this excerpt from carl sandburg's "how the animals lost their tails and got them back traveling from philadelphia to medicine rock"? it was easy enough for the rabbit who has long ears and no tail at all except a white thumb of cotton. but it was hard for the yellow flongboo who at night lights up his house in a hollow tree with his fire yellow torch of a tail. it is hard for the yellow flongboo to lose his tail because it lights up his way when he sneaks at night on the prairie, sneaking up on the flangwayers, the hippers and hangjasts, so good to eat.