For the TOS episode with a similar title, please see "What Are Little Girls Made Of?".
"What Are Little Boys Made Of?" was an old Earth nursery rhyme that cited that "girls are made from sugar and spice and boys are made from snips and snails."
William T. Riker referred to the poem in 2368, during a discussion in Ten Forward with the androgynous J'naii Soren. Upon hearing the rhyme, Soren interpreted the words to mean that it "makes it sound better to be female." Riker then clarified that "it's an old-fashioned way of looking at the sexes, not to say that there's no real difference between them." (TNG: "The Outcast")