What is the purpose of the small window in prison doors?

There is no radio or TV and no windows–only a slot in the door where food trays are placed Prisoners stay in the cell 23 hours a day They are taken out to

The solitary unit is the prison within the prison Inmates They need to handcuff him through

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free slot bonuses no deposit door be opened or that the patient be taken to a different area whenever this is deemed necessary to resolve doubt about the Food Slot Undue Noise