OMG WHY DID THE CAR JUST FALL BACK A LITTLE EVEN THO YOU PARKED IT?!
Well Jake, it's because of gravity. Gravity is a scientific force that causes everything to be attracted to it. On Earth, gravity is constantly pulling us down, towards its center. Since the car was parked on a hill, there is space to be pulled down. Because of this, gravity tried to pull it back down, which made the car do downhill a bit!
But, the wheels on the car are so heavy, wouldn't they stop the car from going down? Besides, the car has 4!
The girl, Sarah, parks the car at an incline, causing the car to lurch back a little. This causes Jake, her brother to freak out because he has never experience this before.
OH THAT MAKES SENSE!!
There are many forces in science, not just gravity! Another force is friction, which is the force that resists motion. There are 4 types of friction, one being rolling friction. This friction acts upon spherical objects and stops them from moving. Rolling friction is also the weakest type of friction. Rolling friction was actually the force that stopped the car from rolling down the hill furthermore, than it did. It's because this force specially resists motion.
Sarah explains to Jake why the car traveled downhill a little bit, despite it being parked.
Um, Sarah? Where'd the car go?
....
Jake is confused because he thinks that the big wheels should've stopped the car from moving.
AHA, MOM AND DAD ARE GONNA BE SO MAD!
WHAT?!
Sarah explains to Jake the factor of rolling friction, and how it relates to the car halting.