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Kate Upton, Bikini

Jackson Lee/Splash News

It's back to work for the birthday girl.

A bikini-wearing Kate Upton, who turned 21 this week, returned to flaunting that bod of hers on the set of The Other Woman—which made the gratuitous cleavage more legit this time by actually including a beach scene in the comedy, about a woman (Cameron Diaz) who finds out that her beau is married and has a much-younger girlfriend.

Guess who plays her.

OK, stop drooling...

Paparazzi caught Upton readjusting the top of her white two-piece as she enjoyed the sunshine in the Hamptons.

Even luckier than those who got an eyeful of the blond bombshell? Her costar Nikolaj Coster-Waldau, who got to perform a kissing scene with the star on set today.

Some people just have all the fun.