The voice is certainly familiar, though he wouldn't have believed that he was just greeted by a five-foot just turtle walking on two legs. There's a momentary stunned silence where Sokka just doesn't say anything. Congratulations, Mikey; you've broken Sokka.

"Thanks?" he says. The implicit question is less about the chalk and more about the shape of the guy that's handing it to him, but that might not be apparent. However, just to clarify things, he does have to know. "You... are the guy(?) that barged into my room that one night, right?"