Friction

Here is a fun game to try.  The goal is to put on the brakes in order to stop a vehicle in a traffic jam.  In it you have a number of different variables (things you can change in the experiment).  You can change the type of vehicle, the speed of the vehicle, the road surface, and the distance at which you apply the brakes.

First try these values:  Big Red Truck, 50 miles per hour, Dry Road, 125 feet away.  You should stop the car.  Now repeat the experiment changing JUST ONE of the values/variables.  What are your results?  Why can't you change more than one thing at a time?