Fred & George Threw Snowballs At Voldemort
In the first book, we are told that Fred and George were pelting snowballs at the back of Quirrell’s turban.
Later on, we come to know that Quirrell was hiding Lord Voldemort inside himself and Voldemort’s face was on the back of Quirrell’s head.
Therefore, Fred and George were throwing snowballs at Voldemort’s face.