Why does Two-Bit grin when Ponyboy is picking up the glass?

In chapter 12 of The Outsiders, Ponyboy, Two-Bit, and Steve are at a grocery store during lunch. While Steve and Two-Bit are inside the store with some girls, Ponyboy is sitting on Steve's car having a cigarette and a Pepsi. A car approaches, and three Socs get out. One of them says to Ponyboy, "We don't like nobody killing our friends, especially Greasers." Ponyboy breaks the end of the bottle off and tells the Socs, "You get back into your car or you'll get split." As he starts to move towards them, the Socs get back in the car and leave.
Two-Bit and Steve come out of the store when the Socs leave. Two-Bit asks Ponyboy, "You really would have used that bottle, wouldn't you?" Two-Bit knows that Ponyboy is different from the rest of them, and he doesn't want to see Ponyboy lose his sensitivity. However, Two-Bit's worries are soon put to rest as Ponyboy picks up the glass from the bottle he has broken. Two-Bit smiles because only Ponyboy would bother to pick up glass from a bottle that is broken to use as a weapon. He realizes that Ponyboy is still different from the rest.