Explanation

The most important part of this sentence comes at the end. If teenagers haven’t been doing anything useful with their time, you need a word that means do nothing useful with your time. Flirt is something teenagers do, but it isn’t useless by definition (it can be quite fun!).

Ruminating can also be useful, as it involves thinking. The correct answer is loaf. Note that the overall tone of this sentence is a bit sarcastic.

Loaf.
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