If you tie a helium-filled balloon inside your car and then accelerate your car all of a sudden, which way will the balloon lean? Post your answers in the comments :) I'll post the answer tomorrow if enough people inquire :P
The balloon will lean foward in the direction you are driving when you accelerate. The reason for this has to do with the difference in density between helium and the surrounding air in the car. To imagine this a little better, imagine you have a sealed cube half filled with liquid and the other half is just air. If you sit that cube in the car and then accelerate the car, the liquid will slosh backwards while the air will go foward. For a more scientific explanation, go here.