The answer to this is that Dally (Dallas Winston) asks to borrow Two-bit's switchblade.
This happens in Chapter 8 of the book. Dally is in the hospital recovering from his injuries that he got when the church burned down.
Two-bit really doesn't want to give it to him, because the knife is Two-bit's pride and joy. But he decides that if Dally is asking for it, it is because he really needs it.
As it turns out, Dally needs it because of the rumble with the Socs that night. He doesn't really need it for the fight itself, but he does use it to "persuade" the nurses to let him out of the hospital.
In "The Outsiders," Chapter 8, (page 133 in my edition) Two-Bit and Ponyboy go to visit Johnny and Dally in the hospital. When Johnny passes out they go to Dally's room. He asks about Johnny and when Two-Bit tells him how bad Johnny is, Dally asks,
"Two-Bit, you still got that fancy black-handled switch?"
"Give it here."
"Two-Bit reached into his back pocket for his prize possession. It was a jet-handled switchblade, ten inches long, that would flash open at a mere breath...Two-Bit handed it over to Dally without a moment's hesitation.
"We gotta win that fight tonight," Dally said, His voice was hard. "We gotta get even with the Socs. For Johnny."
Later Dally tells Ponyboy that he got out of the hospital because the nurse let him leave. He implied that he had threatened her with the "switch."
Dallas Winston borrows Two-bits switchblade while he is in the hospital recovering from the injuries he sustained in the fire. He uses it as a tool to get out of the hospital because he wants to join the rest of the Greasers in the planned rumble. After fighting in the rumble, Dally and Ponybody return to the hospital where Johnny is dying. After his death, Dally leaves the hospital himself and sets himself up for his own death. In that way, you can say that him borrowing Two-bits switchblade ultimately caused his death.
Dally asks for Two Bit's switchblade because he planned to use it to get out of the hospital (threaten doctor/nurse) and take part in the rumble. The author, S.E. Hinton hints this when Dally puts the blade under his pillow.
what is one answer the author hints at?