I'm a riddle in nine syllables,
An elephant, a ponderous house,
A melon strolling on two tendrils.
O red fruit, ivory, fine timbers!
Plath (or the speaker of her poem)* starts off by explaining her method or approach to the poem. She’s a riddle, therefore, she’s poking fun at herself for some reason. Not only is she a riddle, but this poem is carefully structured to require ingenuity in ascertaining its answer. 
 Note the number 9. There are 9 lines and 9 syllables in each line (even the title “Metaphors” has 9 letters!). Clearly, Plath wants us to pay attention to this number…but why? Since we can discern [RIDDLE SPOILER] that the speaker is pregnant, the number relates to the 9 months of pregnancy. The original title was  “Metaphors: A Riddle for Expectant Mothers”. 
 *The poet isn’t the same as the speaker of a poem; like novelists, many poets take radical departures from themselves in the first-person “I"s they create. Ezra Pound, for example, wrote an entire collection called Personae (masks). On the other hand, Plath, along with her cohorts in the Confessional generation of poets, became famous for blurring the lines between her personal life and her public art. "Metaphors” was written  in 1960, the same year  as the birth of her first child, so she may well have had herself in mind as she wrote this riddle.
Metaphors