Good point Pav. Of course
Christ was most likely speaking Aramaic when he renamed
Simon, so
Petros would have been a translation on the part of Greek-speaking disciples, or else the gospel-writers.
I don't think we will ever really know why
Simon got his nickname (maybe he collected stones?), but the explanation given in
Matthew 16:18 seems pretty clear to me. Even if, as
Pavlos and Anneza said, it was just a pun.