detention, dentition

Detention or detention means keeping waiting or keeping in custody, or a term or sentence of time in custody  --
He has to serve detention, for fighting in school.
He has three days detention, after school next week.

Dentition or dentition means teething, as a baby or child cutting teeth or getting new teeth, or the arrangement and number of teeth in general --
Dentition may be somewhat uncomfortable for the infant.
Notice the dentition; this is the skull of a cat.

Now that you know that, you can say --
"He sat in detention reading a book that someone had left behind, about dentition in mammals."