Earplugs Would Be Cruel, Right?

Summer is here.  The smell of sun tan lotion fills the air.  That... and the sound of my 4 year old's non-stop chatter.  It is one big run on sentence around here, and my ears are feeling... weak.  My two current personal favorites are when he says, "Mommy (or Mom, which he is sometimes now calling me - wah!), I have to tell you something".  Then, I sit, waiting expectantly while he tries to remember, or get out whatever supremely important thing he had to tell me at that exact moment.  "Uh... ummm... well... I...uh..."  Then there are "the jokes".  The jokes go on and on, something like this...
"Knock Knock?"
"Who's there?"
"Duck!"
"Duck who?"
"The duck said 'Quack'!"
"Ummm.... okay".
"That's funny, right?"
Now, I have 3 choices.  One, laugh hysterically and act like that is the funniest thing I ever heard.  The problem with this option is that it will only make him tell me more ridiculous jokes.   Two, give a soft chuckle.  The problem with this option is that it will make him want to try harder to get a bigger laugh, which will make him tell me more ridiculous jokes.  Three, say, "That's not really funny".  The problem with this option is that he will be even more determined to make me laugh, so he will tell me more ridiculous jokes.  Help me please.

I'm going to need to sign him up for swim lessons very soon.  You can't talk underwater, right?