Don't encourage them. Those are the barracks' clowns.
3 seconds sound clip from the Stalag 17 movie soundboard.
You can hear this line at 00:53:21.474 in the Blu-ray version of the movie.
Quote context
[...]
- Tea is being served on the verandah.
- Animal, where are the napkins?
- Animal! How many times have I told you, you always gotta pour from the left?
- Don't encourage them. Those are the barracks' clowns.
- Where'd they get you, lieutenant? Over Frankfurt?
- How many ships did you lose?
- Yes. Waddington. 92nd bomber group.
- Hey, lieutenant, how'd you blow up that train with 50 guards around?
- Well, I, heh... Just lucky, I guess.
- Don't let him kid you. Cagney couldn't have pulled a sweeter job.
[...]