How does Ponyboy get a concussion?

Ponyboy gets a concussion during the rumble between the greasers and the Socs in The Outsiders.

He goes into the fight already feeling sick and weak, and he secretly takes five aspirin so he can still join the greasers. The fight itself is brutal, and Ponyboy takes numerous injuries. He is nearly passing out when he hears that the Socs have run away and the greasers have won.

Afterward, Ponyboy goes with Dally to the hospital to see Johnny, then witnesses Johnny’s death and Dally’s death. His dizziness and weakness get worse through all of this until he finally passes out. When he wakes up at home, Darry and Sodapop tell him that he had suffered a concussion during the rumble and had been unconscious for three days. The concussion blurs Ponyboy’s physical injury with his emotional shock. After the rumble and his friends' deaths, Ponyboy's body and mind both struggle to recover.