Her brain begins the encoding process as she hears the sounds of her rowdy classmates from the hallway.
As she walks into the classroom she sees her best friend Becca.
Sarah greets Becca and focuses her attention on listening to Becca recall her eventful weekend.
Becca spends about 45 minutes telling (it was study hall, so they didn't have to worry about paying attention to class) Sarah about how her grandfather got into a car crash over the weekend and is currently in the hospital. Becca was extremely sad and Sarah was able to help her get through that hard time in her life.
Now, everytime Sarah arrives at school and walks into the classroom to see Becca she gets a flashbulb memory. She remembers the time she was sitting in class with becca and spent a long time talking and helping Becca deal with such a hard time in her life.