It will not produce the usual yawn sound.
Nor will you have gotten the yawn out.
The sound the yawn does make will make you laugh.
And the sound of your laugh is also funny.
And you still have that yawn in you.
So your body tries to yawn again, but instead you just get this funny noise.
And twenty minutes later you realize you've been laughing at yourself by yourself.
Which is kinda funny too. But not in a way that makes any noise.