It’s a chilly October night — so cold that my breath is a cloud of steam — and I’m going swimming outside in Paris. That’s right: It’s now possible to do laps en plein air, year-round, in the French capital. I take the plunge and join the late-night swimmers in the Butte-aux-Cailles neighborhood. Floating on my back in the glowing turquoise pool, I can actually see stars in the sky above and peer into apartment windows where residents sip wine over simmering stovetops. Teeth chattering — but totally energized — when I sprint for my towel, I share a laugh with the grinning lifeguard, who is wrapped in a puffy jacket. “Just wait for January!” he says.