Tom creates chaos by playing with a bug and a dog in church.

Tom does not like church much.  He trades for Sunday School tickets instead of memorizing verses.  In order to keep him under control, he sits on the aisle “in order that he might be as far away from the open window and the seductive outside summer scenes as possible” (Ch. 5).  This does not save him from creating a commotion.

Sitting in church trying not to be bored, Tom remembers that he has a bug in his pocket.  Tom keeps his “pinchbug” in a “percussion-cap box” (Ch. 5).  When the insect inevitably bites him, he flicks it away and it lands on its back.  There, it serves as entertainment for the congregants as they try to keep awake in the boring sermon.

That might have been all right, but then a dog wanders in.  This “vagrant poodle dog” spies the beetle, and soon enough it gets bitten too.  The dog plays with the bug awhile, then forgets about it and sits down—right on the beetle.  The dog yelps, runs into its master’s lap, and then its master throws it out the window.  In the end, at least Tom is happy.

Tom Sawyer went home quite cheerful, thinking to himself that there was some satisfaction about divine
service when there was a bit of variety in it. (Ch. 5)

This incident demonstrates Tom’s good nature and his potential for mischief.  More importantly, it is a perfect example of Twain’s satire.  The church service is dull and badly delivered, and no one gets much out of it.  They go for appearances sake, even though a beetle is more interesting than the sermon.