Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you. And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost: John 20:21-22
We should perhaps bear in mind that in Greek the word we translate “Spirit” also means “breath” or “wind.” It was thus very appropriate that Jesus should use breath as an outward sign of the gift of the Spirit that he was giving the disciples.
Sometimes Christians have taken this kind of thing very literally. R.E. Brown tells of a time when the Coptic Patriarch in Alexandria used to breath into a skin bag, which was then firmly tied and taken up river to Ethiopia. There it was released on the man chosen to be head of the Ethiopian church!
We cannot think that Jesus meant anything like this to happen, but the custom shows how suitable it was in the ancient world to use breath to convey teaching about the Spirit.
(Morris, Leon. 2000. Reflections on the Gospel of John. Peabody, MA: Hendrickson Publishers., pg 710)