‘Now you’ve asked for it, and I’ll give it to you, because there ain’t anything mean about me; but if you find you don’t like it, you mustn’t blame anybody but your own self.’

     Peter was agreeable. So Tom opened his mouth and poured down the pain-killer. Peter sprang a couple of yards in the air, and then delivered a war-whoop and set off round and round the room, banging against furniture, upsetting flower-pots, and making general confusion. Next he rose on his hind feet and danced around, in a frenzy of enjoyment, with his head over his shoulder and his voice proclaiming his happiness. Then he went tearing around the house again spreading chaos and destruction in his path. Aunt Polly entered in time to see him throw a few double summersets, deliver a final mighty hurrah, and sail through the open window, carrying the rest of the flower-pots with him. The old lady stood petrified with astonishment, peering over her glasses; Tom lay on the floor expiring with laughter.

     ‘Tom, what on earth ails that cat?’

     ‘I don’t know, aunt,’ gasped the boy.

     ‘Why, I never see anything like it. What did make him act so?’

     ‘Deed I don’t know, Aunt Polly; cats always act so when they’re having good time.’
     ‘They do, do they?’ There was something in the tone of that made Tom apprehensive.

     ‘Yes’m. That is, I believe they do.’

     ‘You do?’

     ‘Yes’ m.’
Tom said to Peter that he is ready to give as it asked for the pain-killer. Tom mentions that if Peter does not like it, then he should not blame anyone other than himself.
Tom was talking to Peter.
Peter agreed for what Tom has said. So, Tom has opened the mouth of Peter and poured the pain killer. Peter reacted in a drastic way he first jumped a couple of yards in the air, cried and then circled the room hitting against the furniture, breaking the flower pots and looked confused. Next, he stood on his back feet and enjoyed dancing in an uncontrolled excitement with his head over his shoulder. The cat's voice came out entirely with happiness. Then the cat crying and tore many things on its way.
Peter rose on his hind feet and danced around.
Aunt Polly entered the room when Peter was doing an acrobatic movement with a complete revolution and finally made a loud noise before jumping throw the open window by breaking the rest of the flower pots. The old lady was frightened by whatever was happening, she adjusted her glasses to understand, and she saw Tom laughing on the floor.
Polly asked Tom what on earth was happened to that cat?
Tom replied that he does not know anything what happened to it.
Aunt said that she never saw Peter react that way anytime.
Tom still said that he has no idea, but cat's do react that way when they are having a good time.
Aunt was not convinced with Tom's reply, but maybe they do.
Tom said that he believes that cat do react that way, but Aunt asked him again if Tom does that way when he is happy, Tom replied that he does.
Meanings of difficult words:
Proclaiming Announce publicly.
SummersetsA forward roll in which the head is placed on the ground.
War-whoop A yell uttered in making an attack.
Petrified Terrified.
State Council of Educational Research and Training (2019). Term-2 English Standard-9. The Cat and the Pain-Killer--(An Extract from The Adventures of Tom Sawyer) Mark Twain pp. 112-115). Published by the Tamil Nadu Textbook and Educational Services Corporation.