Complex sentence: After some severe stomach pains from the pizza, the three head back, they forget Lily at a rest stop, but she clings to the top of the car using the power of their caffeinated soda.
Split into simpler sentences: After some severe stomach pains from the pizza, the three head back and think they forgot Lily at a rest stop. However, she has clung to the top of the car using the power of their caffeinated soda.