Theory:

The poem "Whatif" is written by Shel Silverstein. It is a light verse where the speaker of the poem asks a series of  'what if' questions to himself.
  
When the speaker was lying and thinking the previous night, a group of 'whatifs' entered his head through the ears. They pranced, partied, and sang their old whatif song. In short, the speaker spent the entire night worrying over various absurd and possible scenarios that may or may not even take place.

The speaker has concerns that are common to students of his age. He is worried that he might be treated as a stupid person in his school or fail in his examination. But he also voices out the concerns faced by young kids, such as being ill-treated at school or elsewhere, hated or ignored by people, parents getting divorced, and such. Moreover, the poem reveals the insecurities of kids, such as whether they would grow taller or their teeth would grow even. Also, the speaker wonders what would happen if he never learns to dance as it has a huge role to play in him becoming popular and also stepping out to adulthood.

On the other hand, the poem also contains questions and concerns that are absurd. Green hair growing on his chest or head getting smaller are some of his pointless worries. However, they also reveal the imaginative side of kids.

Hence, a big list of 'whatifs' runs through the speaker's mind. As the night breaks into dawn, the whatif song also comes to an end. The speaker seems happy and relieved, but unfortunately, the 'whatif'-thoughts reappear during the night to keep him restless.
 
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The poem contains a series of questions beginning with whatifs