Here's our act, in full. The twenty-something cashier is the willing, or possibly unwilling, participant from the audience.
Cashier: "You've got your hands-full!" as he watches Ziggy helpfully pile the groceries on the counter, dropping every-other-one.
Me: "Oh no, this is nothing! I've only got the one with me. There's five more at home!"
Cashier gives me a wide-eyed look, a little too wide-eyed. I worry I've traumatized the twenty-something cashier with my joke about having six kids.
Me (contritely): "That's totally not true. I'm kidding."
Ziggy: "Yeah, that's totally not true, 'cause there's ten more at home!"
And when someone improvs like that, you don't ruin the joke.
You just wave, smile and say goodbye to the truly now wide-eyed cashier.