You have probably heard this one before, but I thought it was pretty clever!
Here it is...
Why will Southwest Airlines never have any babies?
--Because they always pull out ontime!
Another variation goes sorta like this...
Well, you know how you always see little baby ducks following their parent ducks around... you will nevert see baby southwest planes trailing behind thir parents, because southwest always pulls out on-time!
(I dont really like that version as much)