Answer:
Step-by-step explanation:
At their fun sleepover, Carrie and Harriet made a meal fro Rebekah from various ingredients that shouldn't be mixed and told her to eat it, because those are the rules. Even if it was disgusting, Rebekah did eat it all, proving she wasn't the coward and that she is ready for the game.
To prove herself even funnier and stronger, she also mixes something for Carrie and Harriet and dares them to drink it.
By doing this, Rebekah proved to be up to challenge and fun, unafraid of making jokes and being the subject of one. This act straightened the friendship of three girls; they have all been tricked by one another, all been part of the game and all participated in the challenge. That is why the text says Rebekah knows they will be friends for a long time.