Kristen Bell [1] is a total pro at hosting late-night television. On Thursday, the actress subbed in for Jimmy Kimmel, who stayed at home to be with his newborn son Billy [2]. After delivering a hilarious opening monologue [3], the mom of two also helped one lucky high school student pull off an incredible Frozen [4]-themed promposal. During a fake game of "The Fast and the Frozenest," one girl in the audience got a huge surprise when the masked Kristoff she was tasked with finding was actually one of her high school classmates. Kristen then channeled her famous character, Anna, to sing a revised version of "Do You Want to Build a Snowman?" to help him ask her to prom. With a grand gesture like that, she couldn't help but say yes.