There’s nothing basic about Sharon Stone’s bikini body — at 59, she’s the envy of women half her age! And an insider says she owes her trim midsection to a strict regimen that includes 300 crunches a day. “She’s been doing them since she started rehearsals for Basic Instinct, back in 1991,” says the source. “It’s not just standard crunches; she mixes in mountain climbers, oblique twists, all kinds of variations. And she says she takes only one day off on weekends.” The mom of three, who’s also devoted to yoga, free weights and a low-carb diet, “has to be extra-militant about her body now that she’s almost 60,” adds the insider. But the work is paying off: The ’90s icon has five new films in the works!