Because you want a body like a ballerina.
Rive Gauche @ 16 Rue De La Grand Chaumiere
Must do the Barre Classes--my two faves are XTend Barre and Cardio Flow Barre. The studio also offers Pilates, Yoga and several machine classes like Reformer and Spinning.
Honestly, you should be going at least a few times a week to live a healthy lifestyle, BUT if you are ever in a bind and need to tone up for a special occasion, a few classes will get you toned and ready to go. First thing in the am or last class in the evening is best during the summer months so you stay cool. When I am on a regular schedule, Tuesday, Thursday and Fridays usually give you a good variety of classes and helps you justify taking the weekend off. :)
Pro-Tip: Definitely take the 8pm Cardio Flow Barre on Thursday night. You won't regret it.
So my FAVE instructor is Antonio for Cardio Flow Barre and XTend Barre (Amazing results + The most fun you will have in a gym class EVER). I mean, I actually saw French women smile and laugh, which is like the 8th wonder of the world! All classes are taught in French and English--the instructor usually switches back and forth, so it is not only good for your body, but good for your brain since you can practice your français!