Her friends cheered her on from the sidelines.
Meaning: The girl's friends were watching her from the side of the puddle and they were shouting and clapping to show their support.
Choice of Words: Friends are people who you know and like and who like you back. Cheered means to shout or clap to show support or approval. Sidelines are the areas next to a sports field where people sit or stand to watch the game.
Alternative Expressions
Related Expressions