A dirty marine joke

Its 5 in the morning. The marine recruits are lined up outside their barracks. Nude. Its mid-January. In Alaska. The sergeant walks up to the first marine and whacks him across the … [fill in the blank].

The sergeant barks: Did you feel that, soldier?

The recruit responds: No, sir!

The sergeant: Why not, soldier?

The recruit: Because Im a rough tough marine, sir!

The sergeant goes to the next marine and whacks him across his … [fill in the blank]. The sergeant bellows: Did you feel that, soldier?

The recruit screams: No, sir!

Why not, soldier? Because Im a rough tough marine. Sir!

The sergeant goes to the third marine and etc. etc. Did you feel that?

No, sir!

Why not?

Because it belongs to the guy behind me!