Anyone who rides a tandem around other people has gotten this comment or something similar hundreds of times. Some of my friends over at Bike Forums have suggested a number of witty responses:
"Who? There's someone on the back?"
"She has to. We're chained together."
"She better, because I'm not."
"Your bike is missing a motor."
"I steer. She pedals."
"Whatever route I take, she's right behind me."
Then there is this sign, which nips the "she's not pedaling" comment in the bud: