Saeril let herself fall, and the surprise of Kili's reaction made her become amused again. She did expect laughter to come from him, but there was a little voice in the back of her head that told her that he would start screaming. 

Before she could hit head-first into the water, the she-elf allowed her wings to expand, and lift herself to fly above the water. Letting Kili see his reflection on the surface of the water, Saeril didn't notice that Fili was watching them from his place where the dwarves and Yavanna remained. He was happy for his brother to experience the impossible, but he didn't want his little brother to get over his head if things went wrong. Nevertheless, the brunette seemed to be having fun, almost mistaking his screaming to be screams of joy.