Complex sentence: Sian and Sophie decide to go to the school prom together but Sian starts getting stomach pains and Sophie, thinking she is trying to get out of going to the prom, argues with Sian.
Split into simpler sentences: Sian and Sophie decide to go to the school prom together. Sian begins getting stomach pains and Sophie, thinking she is trying to escape going to the prom with her, angrily falls out with Sian.