Q: Why do prisoners like the punctuation marks period (.), exclamation point (!) and question mark (?) when they write?
A: Because each one marks the end of their sentence!

Explanation:  If you have been convicted of a crime and sent to jail, your sentence is the time that you must stay in jail.  Of course, everyone in jail awaits the end of their sentence so they can get out of jail.

The punctuation marks period, exclamation point and question mark are used to indicate the end of a sentence when writing.  In writing, a sentence is a complete thought that can stand alone, typically including at least a subject and a verb (but there are exceptions).

This joke is funny because it plays with two meanings of the word sentence.

Speaking of sentences, watch this video for information about a different type of sentence, an interjection.