There are actually two different reasons for barfing.
One is the confusion between inner ear and eye.
Anything which you look at thats moving in a way that disagrees with your inner ear will make you barf. Looking at the inside of your eye lids will also settle your stomach.
The other is to do with natural frequencys of bits of your bodies.
Young childrens natural frequency of thier stomachs unfortunately are on one of the harmonics of cars suspension systems. As they get older and bigger the natural frequency changes and they don't barf any more. You can gig the stiffness of your suspension springs on the car and get rid of it that way as well.