The next week passed swiftly. I expected Kelly to hector me the whole time, but instead, she acted like nothing had happened. Again, I wondered if maybe she'd decided to forget the whole thing. But when Sunday rolled back around, she spent the whole morning reminding me that we were skipping our usual brunch so that I could give her all my money.

"You're an idiot," she told me, flat out.

"We'll see," I said, giving her a playful wink.

The intervening time had given me time to collect my supplies, but also to seriously rethink what I'd agreed to. Not the bet, exactly. I was strangely confident I'd win,...