There are some things girls are willing to share. A kill-it-on-the-spot zit cream. Butt-shrinking jeans. Even a shrink’s number. But when it comes to scents, the ex-boyfriend rule applies: Don’t even think about it. Canadian perfurmier Susanne Lang bases her line on a mix-and-match ideology — so odds are that the combo you choose won’t be the same as your pallie’s. Light scents like cashmere, Anjou pear, and vanilla coconut are meant to be lathered, spritzed, and layered until you find the perfect mix. You select the application method — from shower gels and lotions to rollers and dry oils (or a combo). No one will ever guess how you came to smell as good as you do. Because the only thing better than getting a secret from a friend is having one of your own. Available at StinkerBelle, 1951 West Division Street, between Damen Avenue and Wolcott Street (773-252-4120).