Because they have a hard time puckering their lips the right way.
If they are thick, they have a hard time to rise
You're really bustin' Machops!
He finishes her drink EDIT: Apparently this is Conan's joke, so all credit goes to him. I just heard it from a friend of mine and I had no idea.
She had mittens on.
She only puckers up for her pacifier.
They are sitting on their lips!
He burned his lips on the exhaust pipe.
Because dogs can't whistle. (X-post from r/dadjokes)